diff --git a/app.py b/app.py
index 5f17d8b..eb48fdf 100644
--- a/app.py
+++ b/app.py
@@ -547,6 +547,31 @@ def analyze(draft: Draft):
             raw_groups[gi]["toks"].append(p)
             grouped.add(id(p))
 
+    # fuse groups that carry the same vowel family — a perfect subgroup
+    # (shoulder/older/colder) shouldn't split colors with the slant family
+    # it lives inside (soldier/holster/coaster). Perfect members keep the
+    # strong styling; the slant side keeps per-token slant marks.
+    by_family: dict[str, dict] = {}
+    fused: list[dict] = []
+    for g in raw_groups:
+        mk = founding_projections(g["key"]).get("multi")
+        tgt = by_family.get(mk) if mk else None
+        if tgt is None:
+            if mk:
+                by_family[mk] = g
+            fused.append(g)
+            continue
+        if g["slant"]:
+            for t in g["toks"]:
+                t["slant"] = True
+        elif tgt["slant"]:
+            for t in tgt["toks"]:
+                t["slant"] = True
+            tgt["slant"] = False
+            tgt["key"] = g["key"]
+        tgt["toks"].extend(g["toks"])
+    raw_groups = fused
+
     # stable colors: order groups by first appearance
     raw_groups.sort(key=lambda g: min((t["line"], t["start"]) for t in g["toks"]))
     groups_out = []
diff --git a/tests/test_rhymes.py b/tests/test_rhymes.py
index 6c4bd23..0a478e6 100644
--- a/tests/test_rhymes.py
+++ b/tests/test_rhymes.py
@@ -281,3 +281,19 @@ def test_annotation_lines_ignored():
     (st,) = res["stanzas"]
     assert st["lines"] == [1, 2, 5, 6]
     assert st["scheme"] == "aabb"
+
+
+def test_perfect_subgroup_fuses_with_slant_family():
+    # shoulder/older/colder (perfect, OW L D ER) live inside the bigger
+    # OW-schwa family — one color for the whole column
+    text = ("Skunk, bug, soldier\n"
+            "Tongue, shrub, shoulder\n"
+            "One month older\n"
+            "Sponge, mob, colder\n"
+            "Nun, rug, holster\n"
+            "Lug nut, coaster\n"
+            "Lung, jug, roaster\n"
+            "Young Thug poster\n"
+            "Unplugged toaster")
+    group_with(text, "soldier", "shoulder", "older", "colder", "holster",
+               "coaster", "roaster", "poster", "toaster")