diff --git a/reachabilityindex/skiplist/src/lib.rs b/reachabilityindex/skiplist/src/lib.rs
index 7f40fde7d6..febc652970 100644
--- a/reachabilityindex/skiplist/src/lib.rs
+++ b/reachabilityindex/skiplist/src/lib.rs
@@ -667,6 +667,8 @@ impl LeastCommonAncestorsHint for SkiplistIndex {
         .boxify()
     }
 
+    /// Check if `ancestor` changeset is an ancestor of `descendant` changeset
+    /// Note that a changeset IS NOT its own ancestor
     fn is_ancestor(
         &self,
         ctx: CoreContext,
diff --git a/reachabilityindex/src/index.rs b/reachabilityindex/src/index.rs
index bf94b6a2e0..502d9f7452 100644
--- a/reachabilityindex/src/index.rs
+++ b/reachabilityindex/src/index.rs
@@ -144,6 +144,8 @@ pub trait LeastCommonAncestorsHint: Send + Sync {
         gen: Generation,
     ) -> BoxFuture<NodeFrontier, Error>;
 
+    /// Check if `ancestor` changeset is an ancestor of `descendant` changeset
+    /// Note that a changeset IS NOT its own ancestor
     fn is_ancestor(
         &self,
         ctx: CoreContext,