diff --git a/draftlogs/7684_change.md b/draftlogs/7684_change.md
new file mode 100644
index 00000000000..93797426c49
--- /dev/null
+++ b/draftlogs/7684_change.md
@@ -0,0 +1 @@
+ - Set default layout.axis.tickmode to 'sync' when axis is overlaying [[#7684](https://github.com/plotly/plotly.js/pull/7684)]
\ No newline at end of file
diff --git a/src/plots/cartesian/axes.js b/src/plots/cartesian/axes.js
index 08bfda6f367..d99867a7e50 100644
--- a/src/plots/cartesian/axes.js
+++ b/src/plots/cartesian/axes.js
@@ -1113,6 +1113,9 @@ axes.calcTicks = function calcTicks(ax, opts) {
             var obj = { value: x };
 
             if(major) {
+                // mark first major tick, for showexponent/showtickprefix/showticksuffix 'first'
+                if(x === x0) obj.first = true;
+
                 if(isDLog && (x !== (x | 0))) {
                     obj.simpleLabel = true;
                 }
@@ -1255,9 +1258,10 @@ axes.calcTicks = function calcTicks(ax, opts) {
         tickVals.pop();
     }
 
-    // save the last tick as well as first, so we can
-    // show the exponent only on the last one
+    // save the last tick as well as first
     ax._tmax = (tickVals[tickVals.length - 1] || {}).value;
+    // mark the last major tick, for showexponent/showtickprefix/showticksuffix 'last'
+    if(tickVals.length) tickVals[tickVals.length - 1].last = true;
 
     // for showing the rest of a date when the main tick label is only the
     // latter part: ax._prevDateHead holds what we showed most recently.
@@ -1279,7 +1283,8 @@ axes.calcTicks = function calcTicks(ax, opts) {
             ax,
             tickVal.value,
             false, // hover
-            tickVal.simpleLabel // noSuffixPrefix
+            tickVal.simpleLabel, // noSuffixPrefix
+            {first: tickVal.first, last: tickVal.last} // positionFlags
         );
         var p = tickVal.periodX;
         if(p !== undefined) {
@@ -1351,6 +1356,16 @@ function syncTicks(ax) {
 
     var ticksOut = [];
     if(baseAxis._vals) {
+        // find the indices of the first and last labelled (major, non-noTick) base ticks,
+        // for showexponent/showtickprefix/showticksuffix 'first'/'last'
+        var firstMajorIdx = -1;
+        var lastMajorIdx = -1;
+        for(var j = 0; j < baseAxis._vals.length; j++) {
+            if(baseAxis._vals[j].noTick || baseAxis._vals[j].minor) continue;
+            if(firstMajorIdx === -1) firstMajorIdx = j;
+            lastMajorIdx = j;
+        }
+
         for(var i = 0; i < baseAxis._vals.length; i++) {
             // filter vals with noTick flag
             if(baseAxis._vals[i].noTick) {
@@ -1362,7 +1377,10 @@ function syncTicks(ax) {
 
             // get the tick for the current axis based on position
             var vali = ax.p2l(pos);
-            var obj = axes.tickText(ax, vali);
+            var obj = axes.tickText(ax, vali, false, undefined, {
+                first: i === firstMajorIdx,
+                last: i === lastMajorIdx,
+            });
 
             // assign minor ticks
             if(baseAxis._vals[i].minor) {
@@ -1408,10 +1426,27 @@ function arrayTicks(ax, majorOnly) {
         // except with more precision to the numbers
         if(!Lib.isArrayOrTypedArray(text)) text = [];
 
+        // indices of the first and last in-range major ticks,
+        // showexponent/showtickprefix/showticksuffix 'first'/'last'
+        var firstIdx = -1;
+        var lastIdx = -1;
+        if(!isMinor) {
+            for(var k = 0; k < vals.length; k++) {
+                var valk = tickVal2l(vals[k]);
+                if(valk > tickMin && valk < tickMax) {
+                    if(firstIdx === -1) firstIdx = k;
+                    lastIdx = k;
+                }
+            }
+        }
+
         for(var i = 0; i < vals.length; i++) {
             var vali = tickVal2l(vals[i]);
             if(vali > tickMin && vali < tickMax) {
-                var obj = axes.tickText(ax, vali, false, String(text[i]));
+                var obj = axes.tickText(ax, vali, false, String(text[i]), {
+                    first: i === firstIdx,
+                    last: i === lastIdx,
+                });
                 if(isMinor) {
                     obj.minor = true;
                     obj.text = '';
@@ -1725,13 +1760,23 @@ axes.tickFirst = function(ax, opts) {
     } else throw 'unrecognized dtick ' + String(dtick);
 };
 
-// draw the text for one tick.
-// px,py are the location on gd.paper
-// prefix is there so the x axis ticks can be dropped a line
-// ax is the axis layout, x is the tick value
-// hover is a (truthy) flag for whether to show numbers with a bit
-// more precision for hovertext
-axes.tickText = function(ax, x, hover, noSuffixPrefix) {
+/**
+ * Compute the text and metadata for one tick.
+ *
+ * @param {object} ax: the axis layout object
+ * @param {number} x: the tick value
+ * @param {boolean} hover: whether tick being computed for hovertext (as opposed to axis)
+ * @param {boolean} noSuffixPrefix: whether to skip adding tickprefix and ticksuffix
+ * @param {object} positionFlags: optional flags describing where this tick sits on the
+ *     axis, used by the showexponent/showtickprefix/showticksuffix 'first'/'last' options:
+ *       - first (boolean): whether this is the first (labelled, major) tick on the axis
+ *       - last (boolean): whether this is the last (labelled, major) tick on the axis
+ * @return {object} the tick object, including its formatted `text`
+ */
+axes.tickText = function(ax, x, hover, noSuffixPrefix, positionFlags) {
+    var first = !!positionFlags?.first;
+    var last = !!positionFlags?.last;
+
     var out = tickTextObj(ax, x);
     var arrayMode = ax.tickmode === 'array';
     var extraPrecision = hover || arrayMode;
@@ -1765,13 +1810,10 @@ axes.tickText = function(ax, x, hover, noSuffixPrefix) {
     function isHidden(showAttr) {
         if(showAttr === undefined) return true;
         if(hover) return showAttr === 'none';
-
-        var firstOrLast = {
-            first: ax._tmin,
-            last: ax._tmax
-        }[showAttr];
-
-        return showAttr !== 'all' && x !== firstOrLast;
+        if(showAttr === 'all') return false;
+        if(showAttr === 'first') return !first;
+        if(showAttr === 'last') return !last;
+        return true; // fallback for the hover is false and showAttr==='none' or another value
     }
 
     var hideexp = hover ?
diff --git a/src/plots/cartesian/dragbox.js b/src/plots/cartesian/dragbox.js
index 0aa0caceee2..e66ec74628d 100644
--- a/src/plots/cartesian/dragbox.js
+++ b/src/plots/cartesian/dragbox.js
@@ -732,7 +732,10 @@ function makeDragBox(gd, plotinfo, x, y, w, h, ns, ew) {
             for(i = 0; i < axList.length; i++) {
                 var axListI = axList[i];
                 var axListIType = axListI[axisType];
-                if(!axListI.fixedrange && axListIType.tickmode === 'sync') activeAxIds.push(axListIType._id);
+                var axId = axListIType._id;
+                if(!axListI.fixedrange && axListIType.tickmode === 'sync' && !activeAxIds.includes(axId)) {
+                    activeAxIds.push(axId);
+                }
             }
         }
 
diff --git a/src/plots/cartesian/layout_attributes.js b/src/plots/cartesian/layout_attributes.js
index d1ce0158ace..d7c207fd21f 100644
--- a/src/plots/cartesian/layout_attributes.js
+++ b/src/plots/cartesian/layout_attributes.js
@@ -35,7 +35,8 @@ var tickmode = extendFlat({}, minorTickmode, {
     description: [
         minorTickmode.description,
         'If *sync*, the number of ticks will sync with the overlayed axis',
-        'set by `overlaying` property.'
+        'set by `overlaying` property. When no other tick info is provided,',
+        'overlaying (non-categorical) axes default to *sync*, while other axes default to *auto*.',
     ].join(' ')
 });
 
diff --git a/src/plots/cartesian/tick_value_defaults.js b/src/plots/cartesian/tick_value_defaults.js
index 68b9207ee62..faa2d36f03b 100644
--- a/src/plots/cartesian/tick_value_defaults.js
+++ b/src/plots/cartesian/tick_value_defaults.js
@@ -5,8 +5,7 @@ var isArrayOrTypedArray = require('../../lib').isArrayOrTypedArray;
 var isTypedArraySpec = require('../../lib/array').isTypedArraySpec;
 var decodeTypedArraySpec = require('../../lib/array').decodeTypedArraySpec;
 
-module.exports = function handleTickValueDefaults(containerIn, containerOut, coerce, axType, opts) {
-    if(!opts) opts = {};
+module.exports = function handleTickValueDefaults(containerIn, containerOut, coerce, axType, opts = {}) {
     var isMinor = opts.isMinor;
     var cIn = isMinor ? containerIn.minor || {} : containerIn;
     var cOut = isMinor ? containerOut.minor : containerOut;
@@ -14,35 +13,40 @@ module.exports = function handleTickValueDefaults(containerIn, containerOut, coe
 
     function readInput(attr) {
         var v = cIn[attr];
-        if(isTypedArraySpec(v)) v = decodeTypedArraySpec(v);
+        if (isTypedArraySpec(v)) v = decodeTypedArraySpec(v);
 
-        return (
-            v !== undefined
-        ) ? v : (cOut._template || {})[attr];
+        return v !== undefined ? v : (cOut._template || {})[attr];
     }
 
     var _tick0 = readInput('tick0');
     var _dtick = readInput('dtick');
     var _tickvals = readInput('tickvals');
+    var _overlaying = readInput('overlaying');
+    var _categorical = axType === 'category' || axType === 'multicategory';
 
-    var tickmodeDefault = isArrayOrTypedArray(_tickvals) ? 'array' :
-        _dtick ? 'linear' :
-        'auto';
+    var tickmodeDefault;
+    if (isArrayOrTypedArray(_tickvals)) {
+        tickmodeDefault = 'array';
+    } else if (_dtick) {
+        tickmodeDefault = 'linear';
+    } else if (_overlaying && !_categorical) {
+        tickmodeDefault = 'sync';
+    } else {
+        tickmodeDefault = 'auto';
+    }
     var tickmode = coerce(prefix + 'tickmode', tickmodeDefault);
 
-    if(tickmode === 'auto' || tickmode === 'sync') {
+    if (tickmode === 'auto' || tickmode === 'sync') {
         coerce(prefix + 'nticks');
-    } else if(tickmode === 'linear') {
+    } else if (tickmode === 'linear') {
         // dtick is usually a positive number, but there are some
         // special strings available for log or date axes
         // tick0 also has special logic
-        var dtick = cOut.dtick = cleanTicks.dtick(
-            _dtick, axType);
-        cOut.tick0 = cleanTicks.tick0(
-            _tick0, axType, containerOut.calendar, dtick);
-    } else if(axType !== 'multicategory') {
+        var dtick = (cOut.dtick = cleanTicks.dtick(_dtick, axType));
+        cOut.tick0 = cleanTicks.tick0(_tick0, axType, containerOut.calendar, dtick);
+    } else if (axType !== 'multicategory') {
         var tickvals = coerce(prefix + 'tickvals');
-        if(tickvals === undefined) cOut.tickmode = 'auto';
-        else if(!isMinor) coerce('ticktext');
+        if (tickvals === undefined) cOut.tickmode = 'auto';
+        else if (!isMinor) coerce('ticktext');
     }
 };
diff --git a/src/traces/carpet/calc_labels.js b/src/traces/carpet/calc_labels.js
index 8b7d5a47045..ce75f71fdfc 100644
--- a/src/traces/carpet/calc_labels.js
+++ b/src/traces/carpet/calc_labels.js
@@ -13,7 +13,10 @@ module.exports = function calcLabels(trace, axis) {
         gridline = gridlines[i];
 
         if(['start', 'both'].indexOf(axis.showticklabels) !== -1) {
-            tobj = Axes.tickText(axis, gridline.value);
+            tobj = Axes.tickText(axis, gridline.value, false, false, {
+                first: i === 0,
+                last: i === gridlines.length - 1,
+            });
 
             extendFlat(tobj, {
                 prefix: prefix,
@@ -32,7 +35,10 @@ module.exports = function calcLabels(trace, axis) {
         }
 
         if(['end', 'both'].indexOf(axis.showticklabels) !== -1) {
-            tobj = Axes.tickText(axis, gridline.value);
+            tobj = Axes.tickText(axis, gridline.value, false, false, {
+                first: i === 0,
+                last: i === gridlines.length - 1,
+            });
 
             extendFlat(tobj, {
                 endAnchor: false,
diff --git a/src/types/generated/schema.d.ts b/src/types/generated/schema.d.ts
index 1e05da351d0..ccfbd77a70a 100644
--- a/src/types/generated/schema.d.ts
+++ b/src/types/generated/schema.d.ts
@@ -12876,7 +12876,7 @@ export interface LayoutAxis {
      * Minimum: 0
      */
     ticklen?: number;
-    /** Sets the tick mode for this axis. If *auto*, the number of ticks is set via `nticks`. If *linear*, the placement of the ticks is determined by a starting position `tick0` and a tick step `dtick` (*linear* is the default value if `tick0` and `dtick` are provided). If *array*, the placement of the ticks is set via `tickvals` and the tick text is `ticktext`. (*array* is the default value if `tickvals` is provided). If *sync*, the number of ticks will sync with the overlayed axis set by `overlaying` property. */
+    /** Sets the tick mode for this axis. If *auto*, the number of ticks is set via `nticks`. If *linear*, the placement of the ticks is determined by a starting position `tick0` and a tick step `dtick` (*linear* is the default value if `tick0` and `dtick` are provided). If *array*, the placement of the ticks is set via `tickvals` and the tick text is `ticktext`. (*array* is the default value if `tickvals` is provided). If *sync*, the number of ticks will sync with the overlayed axis set by `overlaying` property. When no other tick info is provided, overlaying (non-categorical) axes default to *sync*, while other axes default to *auto*. */
     tickmode?: 'auto' | 'linear' | 'array' | 'sync';
     /** Sets a tick label prefix. */
     tickprefix?: string;
diff --git a/test/image/baselines/20.png b/test/image/baselines/20.png
index 230a6baf8b7..374551b3b33 100644
Binary files a/test/image/baselines/20.png and b/test/image/baselines/20.png differ
diff --git a/test/image/baselines/autorange-tozero-rangemode.png b/test/image/baselines/autorange-tozero-rangemode.png
index ebc57f4d8d2..138d26a311b 100644
Binary files a/test/image/baselines/autorange-tozero-rangemode.png and b/test/image/baselines/autorange-tozero-rangemode.png differ
diff --git a/test/image/baselines/candlestick_double-y-axis.png b/test/image/baselines/candlestick_double-y-axis.png
index 077f3b8fab0..fd0c83a5331 100644
Binary files a/test/image/baselines/candlestick_double-y-axis.png and b/test/image/baselines/candlestick_double-y-axis.png differ
diff --git a/test/image/baselines/candlestick_rangeslider_thai.png b/test/image/baselines/candlestick_rangeslider_thai.png
index f40e1cdd0ef..5057aaa7197 100644
Binary files a/test/image/baselines/candlestick_rangeslider_thai.png and b/test/image/baselines/candlestick_rangeslider_thai.png differ
diff --git a/test/image/baselines/legend_scroll_beyond_plotarea.png b/test/image/baselines/legend_scroll_beyond_plotarea.png
index 677421dd0ff..51ecafd947b 100644
Binary files a/test/image/baselines/legend_scroll_beyond_plotarea.png and b/test/image/baselines/legend_scroll_beyond_plotarea.png differ
diff --git a/test/image/baselines/legend_visibility.png b/test/image/baselines/legend_visibility.png
index e80069a9c02..bbdb2156d96 100644
Binary files a/test/image/baselines/legend_visibility.png and b/test/image/baselines/legend_visibility.png differ
diff --git a/test/image/baselines/mult-yaxes-subplots-stacked.png b/test/image/baselines/mult-yaxes-subplots-stacked.png
index 5aac4b389cd..855dcde1bf3 100644
Binary files a/test/image/baselines/mult-yaxes-subplots-stacked.png and b/test/image/baselines/mult-yaxes-subplots-stacked.png differ
diff --git a/test/image/baselines/multicategory_series.png b/test/image/baselines/multicategory_series.png
index dfbe24abe2e..7e4b0c8edf0 100644
Binary files a/test/image/baselines/multicategory_series.png and b/test/image/baselines/multicategory_series.png differ
diff --git a/test/image/baselines/range_slider_legend_left.png b/test/image/baselines/range_slider_legend_left.png
index 9eb5c4457ff..d35f8c99b28 100644
Binary files a/test/image/baselines/range_slider_legend_left.png and b/test/image/baselines/range_slider_legend_left.png differ
diff --git a/test/image/baselines/yaxis-over-yaxis2.png b/test/image/baselines/yaxis-over-yaxis2.png
index 175f1c39adb..6d3398ef7aa 100644
Binary files a/test/image/baselines/yaxis-over-yaxis2.png and b/test/image/baselines/yaxis-over-yaxis2.png differ
diff --git a/test/image/baselines/zerolinelayer_above.png b/test/image/baselines/zerolinelayer_above.png
index 10fca3a7e8c..6e4490680da 100644
Binary files a/test/image/baselines/zerolinelayer_above.png and b/test/image/baselines/zerolinelayer_above.png differ
diff --git a/test/image/baselines/zerolinelayer_below.png b/test/image/baselines/zerolinelayer_below.png
index d011848fa65..9e43516c183 100644
Binary files a/test/image/baselines/zerolinelayer_below.png and b/test/image/baselines/zerolinelayer_below.png differ
diff --git a/test/image/mocks/shapes_layer_below_traces.json b/test/image/mocks/shapes_layer_below_traces.json
index 9b221568538..049b66b9c4c 100644
--- a/test/image/mocks/shapes_layer_below_traces.json
+++ b/test/image/mocks/shapes_layer_below_traces.json
@@ -103,7 +103,8 @@
     "yaxis2": {
       "gridwidth": 2,
       "side": "right",
-      "overlaying": "y"
+      "overlaying": "y",
+      "tickmode": "auto"
     }
   }
 }
diff --git a/test/jasmine/tests/axes_test.js b/test/jasmine/tests/axes_test.js
index 3d1e962312f..5a9457f9ad2 100644
--- a/test/jasmine/tests/axes_test.js
+++ b/test/jasmine/tests/axes_test.js
@@ -8416,4 +8416,33 @@ describe('test tickmode calculator', function() {
             }).then(done, done.fail);
         });
     });
+
+    describe('sync', function() {
+        it('shows the exponent on a synced overlaying axis with showexponent *first*/*last*', function(done) {
+            Plotly.newPlot(gd, {
+                data: [
+                    {y: [0, 6]},
+                    {y: [0, 60000], yaxis: 'y2'}
+                ],
+                layout: {
+                    yaxis2: {
+                        overlaying: 'y',
+                        exponentformat: 'SI',
+                        showexponent: 'last'
+                    }
+                }
+            }).then(function() {
+                var ax = gd._fullLayout.yaxis2;
+                expect(ax.tickmode).toBe('sync');
+
+                var labels = ax._vals
+                    .filter(function(d) { return !d.minor; })
+                    .map(function(d) { return d.text; });
+
+                // the multiplier (e.g. 'k') must still appear on the labelled tick
+                expect(labels.some(function(t) { return /k$/.test(t); }))
+                    .toBe(true, 'expected an SI prefix in: ' + JSON.stringify(labels));
+            }).then(done, done.fail);
+        });
+    });
 });
diff --git a/test/plot-schema.json b/test/plot-schema.json
index 86964d4f5e9..5ae14291f41 100644
--- a/test/plot-schema.json
+++ b/test/plot-schema.json
@@ -14867,7 +14867,7 @@
           "valType": "number"
         },
         "tickmode": {
-          "description": "Sets the tick mode for this axis. If *auto*, the number of ticks is set via `nticks`. If *linear*, the placement of the ticks is determined by a starting position `tick0` and a tick step `dtick` (*linear* is the default value if `tick0` and `dtick` are provided). If *array*, the placement of the ticks is set via `tickvals` and the tick text is `ticktext`. (*array* is the default value if `tickvals` is provided). If *sync*, the number of ticks will sync with the overlayed axis set by `overlaying` property.",
+          "description": "Sets the tick mode for this axis. If *auto*, the number of ticks is set via `nticks`. If *linear*, the placement of the ticks is determined by a starting position `tick0` and a tick step `dtick` (*linear* is the default value if `tick0` and `dtick` are provided). If *array*, the placement of the ticks is set via `tickvals` and the tick text is `ticktext`. (*array* is the default value if `tickvals` is provided). If *sync*, the number of ticks will sync with the overlayed axis set by `overlaying` property. When no other tick info is provided, overlaying (non-categorical) axes default to *sync*, while other axes default to *auto*.",
           "editType": "ticks",
           "impliedEdits": {},
           "valType": "enumerated",
@@ -16146,7 +16146,7 @@
           "valType": "number"
         },
         "tickmode": {
-          "description": "Sets the tick mode for this axis. If *auto*, the number of ticks is set via `nticks`. If *linear*, the placement of the ticks is determined by a starting position `tick0` and a tick step `dtick` (*linear* is the default value if `tick0` and `dtick` are provided). If *array*, the placement of the ticks is set via `tickvals` and the tick text is `ticktext`. (*array* is the default value if `tickvals` is provided). If *sync*, the number of ticks will sync with the overlayed axis set by `overlaying` property.",
+          "description": "Sets the tick mode for this axis. If *auto*, the number of ticks is set via `nticks`. If *linear*, the placement of the ticks is determined by a starting position `tick0` and a tick step `dtick` (*linear* is the default value if `tick0` and `dtick` are provided). If *array*, the placement of the ticks is set via `tickvals` and the tick text is `ticktext`. (*array* is the default value if `tickvals` is provided). If *sync*, the number of ticks will sync with the overlayed axis set by `overlaying` property. When no other tick info is provided, overlaying (non-categorical) axes default to *sync*, while other axes default to *auto*.",
           "editType": "ticks",
           "impliedEdits": {},
           "valType": "enumerated",