diff --git a/CLAUDE.md b/CLAUDE.md
index 2dcdda8..826965a 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -108,7 +108,10 @@ pytest                            # 25 tests
    the pending change over the committed feature; **Apply** commits it
    (`controller.set_preview`/`apply_preview`, `preview_changed` signal →
    `viewport.set_preview`). Per-kind hints + field tooltips explain the
-   parameters. `controller.active_feature` is the one being edited. Not yet:
+   parameters. `controller.active_feature` is the one being edited. A **Fit to
+   mate…** button (`controller.fit_feature`) resizes a mortise to a chosen tenon
+   (or vice versa) — pocket = tongue + clearance (1/32"), pocket slightly deeper;
+   a dialog lists the complementary features (`features_of_kind`). Not yet:
    countersinks, and click-a-face-to-place in the GUI.
 2. **Latency** ~7–13s per utterance (one `claude -p` call).
 3. Voice path (`--voice`) reuses `dictate`; the driver loop is hardened against
diff --git a/src/woodshop/gui/controller.py b/src/woodshop/gui/controller.py
index b721195..5235b4e 100644
--- a/src/woodshop/gui/controller.py
+++ b/src/woodshop/gui/controller.py
@@ -257,6 +257,34 @@ class Controller(QObject):
         except SceneError:
             return None
 
+    def features_of_kind(self, kind: str) -> list[tuple]:
+        """All (part, feature) of a kind, excluding the one being edited."""
+        return [(p, f) for p in self.scene.parts for f in p.features
+                if f.kind == kind and f.id != self.active_feature]
+
+    def fit_feature(self, target_fid: str, clearance: float = 1 / 32) -> None:
+        """Resize the active feature to mate with another (tenon<->mortise)."""
+        feat = self.active_feature_obj()
+        if not feat:
+            return
+        try:
+            _, target = self.scene.find_feature(target_fid)
+        except SceneError:
+            return
+        if feat.kind == "mortise" and target.kind == "tenon":
+            dims = dict(width_in=target.width_in + clearance,
+                        height_in=target.height_in + clearance,
+                        depth_in=target.depth_in + clearance)  # pocket slightly deeper
+        elif feat.kind == "tenon" and target.kind == "mortise":
+            dims = dict(width_in=max(target.width_in - clearance, 0.05),
+                        height_in=max(target.height_in - clearance, 0.05),
+                        depth_in=target.depth_in)               # tongue reaches the bottom
+        else:
+            self.logged.emit("sys", "Fit a mortise to a tenon (or a tenon to a mortise).")
+            return
+        self.scene.edit_feature(feat.id, **dims)
+        self._commit(f"Fitted {feat.id} to {target_fid}.")
+
     # ----- live preview of a pending feature edit ----------------------
     def set_preview(self, **fields) -> None:
         """Stash a pending edit (does NOT change the model) and redraw the ghost."""
diff --git a/src/woodshop/gui/feature_panel.py b/src/woodshop/gui/feature_panel.py
index 17dd036..43f7ad9 100644
--- a/src/woodshop/gui/feature_panel.py
+++ b/src/woodshop/gui/feature_panel.py
@@ -4,8 +4,9 @@ clicking a kind drops a sensibly-sized feature you can immediately adjust."""
 from __future__ import annotations
 
 from PySide6.QtCore import Qt
-from PySide6.QtWidgets import (QComboBox, QDoubleSpinBox, QFormLayout, QGridLayout,
-                               QHBoxLayout, QLabel, QListWidget, QListWidgetItem,
+from PySide6.QtWidgets import (QComboBox, QDialog, QDialogButtonBox, QDoubleSpinBox,
+                               QFormLayout, QGridLayout, QHBoxLayout, QLabel,
+                               QListWidget, QListWidgetItem, QMessageBox,
                                QPushButton, QVBoxLayout, QWidget)
 
 from ..scene import FACES
@@ -61,6 +62,11 @@ class FeaturePanel(QWidget):
         form.insertRow(0, "Face", self.face)
         root.addLayout(form)
 
+        self.fit_btn = QPushButton("Fit to mate…")
+        self.fit_btn.setToolTip("Resize this tenon/mortise to fit a matching one on another board")
+        self.fit_btn.clicked.connect(self._fit)
+        root.addWidget(self.fit_btn)
+
         btns = QHBoxLayout()
         self.apply_btn = QPushButton("Apply")
         self.apply_btn.setToolTip("Commit the previewed change (cuts/adds the real geometry)")
@@ -108,6 +114,9 @@ class FeaturePanel(QWidget):
                 sp.setValue(getattr(feat, key))
         self.hint.setText(_HINTS.get(feat.kind, "") if feat else
                           "Add a feature above, then adjust it here.")
+        mate = {"tenon": "mortise", "mortise": "tenon"}.get(feat.kind if feat else "")
+        self.fit_btn.setEnabled(bool(mate))
+        self.fit_btn.setText(f"Fit to {mate}…" if mate else "Fit to mate…")
         self._loading = False
         if self.c.preview is not None:   # committed state -> drop any stale ghost
             self.c.clear_preview()
@@ -126,6 +135,34 @@ class FeaturePanel(QWidget):
         dims = {key: sp.value() for key, sp in self._spins.items()}
         self.c.set_preview(face=self.face.currentText(), **dims)
 
+    def _fit(self) -> None:
+        feat = self.c.active_feature_obj()
+        mate = {"tenon": "mortise", "mortise": "tenon"}.get(feat.kind if feat else "")
+        if not mate:
+            return
+        cands = self.c.features_of_kind(mate)
+        if not cands:
+            QMessageBox.information(self, "Fit", f"No {mate} on another board to fit to.")
+            return
+        dlg = QDialog(self)
+        dlg.setWindowTitle(f"Fit {feat.kind} to a {mate}")
+        lay = QVBoxLayout(dlg)
+        lay.addWidget(QLabel(f"Select the {mate} to mate with:"))
+        lst = QListWidget()
+        for part, f in cands:
+            label = f"{part.id} · {f.id}: {f.kind} {f.width_in:g}×{f.height_in:g}×{f.depth_in:g}"
+            item = QListWidgetItem(label)
+            item.setData(Qt.UserRole, f.id)
+            lst.addItem(item)
+        lst.setCurrentRow(0)
+        lay.addWidget(lst)
+        bb = QDialogButtonBox(QDialogButtonBox.Ok | QDialogButtonBox.Cancel)
+        bb.accepted.connect(dlg.accept)
+        bb.rejected.connect(dlg.reject)
+        lay.addWidget(bb)
+        if dlg.exec() and lst.currentItem():
+            self.c.fit_feature(lst.currentItem().data(Qt.UserRole))
+
 
 _HINTS = {
     "tenon": "Tenon — a tongue on the chosen end. Width × Height = its cross-section; "
diff --git a/tests/test_gui_controller.py b/tests/test_gui_controller.py
index 19cfa30..809ae2f 100644
--- a/tests/test_gui_controller.py
+++ b/tests/test_gui_controller.py
@@ -99,6 +99,20 @@ def test_feature_preview_mesh_builds():
     assert feature_preview_mesh(s.get_part("p1"), feat).n_points > 0
 
 
+def test_fit_mortise_to_tenon(tmp_path):
+    c = _controller(tmp_path)
+    c.place("2x4", 24)
+    c.add_feature("tenon")                       # f1 on p1, active
+    c.scene.edit_feature("f1", width_in=1.0, height_in=0.75, depth_in=1.5)
+    c.place("2x4", 24)
+    c.add_feature("mortise")                      # f2 on p2, now active
+    c.fit_feature("f1")                           # fit the mortise to the tenon
+    _, m = c.scene.find_feature("f2")
+    assert m.width_in == 1.0 + 1 / 32             # pocket = tongue + clearance
+    assert m.height_in == 0.75 + 1 / 32
+    assert m.depth_in == 1.5 + 1 / 32
+
+
 def test_unknown_tool_is_safe(tmp_path):
     c = _controller(tmp_path)
     assert "unknown" in c.execute_call("wood-bogus", {}).lower()