diff --git a/TODO.md b/TODO.md
index e00e606..19d955b 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,6 +1,5 @@
 # lattice
 
-`lattice/conceptBelow`
 `lattice/conceptAbove`
 
 `LatticeLayout`
diff --git a/src/lattice/conceptBelow.ts b/src/lattice/conceptBelow.ts
index b6001f6..1376bf9 100644
--- a/src/lattice/conceptBelow.ts
+++ b/src/lattice/conceptBelow.ts
@@ -1,7 +1,9 @@
 import { assertSameContext } from "../concept/assertSameContext.js"
 import type { Concept } from "../concept/Concept.js"
+import { setIsSubsetOf } from "../utils/Set.js"
 
 export function conceptBelow(x: Concept, y: Concept): boolean {
   assertSameContext([x, y], { who: "conceptBelow" })
-  return false
+
+  return setIsSubsetOf(x.extent, y.extent)
 }