diff --git a/i2/__init__.py b/i2/__init__.py
index e7ecdf73..b784d57b 100644
--- a/i2/__init__.py
+++ b/i2/__init__.py
@@ -8,6 +8,8 @@
     input_output_decorator,
     wrap_class_methods_input_and_output,
     double_up_as_factory,
+    NotSet,  # Sentinel for "no value given" (distinct from None and Parameter.empty)
+    is_not_set,  # Test whether a value (e.g. a signature default) is NotSet
 )
 
 from i2.signatures import (
diff --git a/i2/deco.py b/i2/deco.py
index 4964828c..8b1e6031 100644
--- a/i2/deco.py
+++ b/i2/deco.py
@@ -55,6 +55,35 @@ def _not_set_repr(self):
 
 
 NotSet = mk_sentinel("NotSet", repr_=_not_set_repr)
+NotSet.__doc__ = """Sentinel meaning "no value was given for this argument".
+
+It is distinct from ``None`` (which can be a legitimate value) and from
+``inspect.Parameter.empty`` (which means "this parameter has no default").
+Code that reads signature defaults should test for it with :func:`is_not_set`
+rather than by comparing to a private object. Note that ``bool(NotSet)`` is
+``False``, so ``if not default:`` would lump it together with ``None``, ``0`` and
+``""``: use :func:`is_not_set` instead.
+"""
+
+
+def is_not_set(x) -> bool:
+    """Return ``True`` if ``x`` is the ``NotSet`` sentinel, and ``False`` otherwise.
+
+    Signature consumers (UI or schema generators, for example) can use it to treat a
+    ``NotSet`` default like ``inspect.Parameter.empty``, i.e. "required, no default":
+
+    >>> from inspect import Parameter
+    >>> is_not_set(NotSet)
+    True
+    >>> is_not_set(None), is_not_set(Parameter.empty), is_not_set("NotSet")
+    (False, False, False)
+    >>> def default_or_empty(param):
+    ...     return Parameter.empty if is_not_set(param.default) else param.default
+    >>> p = Parameter('x', Parameter.KEYWORD_ONLY, default=NotSet)
+    >>> default_or_empty(p) is Parameter.empty
+    True
+    """
+    return x is NotSet
 
 
 # ---------------------------------------------------------------------------------------
diff --git a/i2/tests/test_deco.py b/i2/tests/test_deco.py
index b7523787..2acafa73 100644
--- a/i2/tests/test_deco.py
+++ b/i2/tests/test_deco.py
@@ -37,3 +37,20 @@ def test_func_factory_signature_keeps_required_params_required():
     # The factory itself can still be called with none to all of the arguments
     assert factory()(1, 2, "a") == [1, 2, "a", "-"]
     assert factory(chk_size=3)(1, name="b") == [1, 3, "b", "-"]
+
+
+def test_not_set_is_exported_and_recognisable():
+    """``NotSet`` and ``is_not_set`` are public, so signature consumers can recognise
+    the sentinel without importing a private object (see i2mint/i2#48)."""
+    import pickle
+
+    import i2
+    from i2.deco import NotSet as deco_not_set
+
+    assert i2.NotSet is deco_not_set
+    assert i2.is_not_set(i2.NotSet)
+    for other in (None, inspect.Parameter.empty, "NotSet", 0, False, object()):
+        assert not i2.is_not_set(other)
+    # Survives pickling as the same object (so identity checks stay valid)
+    assert pickle.loads(pickle.dumps(i2.NotSet)) is i2.NotSet
+    assert repr(i2.NotSet) == "NotSet"