diff --git a/src/halfvec.h b/src/halfvec.h
index 99ed3fd..cb021a6 100644
--- a/src/halfvec.h
+++ b/src/halfvec.h
@@ -39,7 +39,7 @@ typedef struct HalfVector
 {
 	int32		vl_len_;		/* varlena header (do not touch directly!) */
 	int16		dim;			/* number of dimensions */
-	int16		unused;
+	int16		unused;			/* reserved for future use, always zero */
 	half		x[FLEXIBLE_ARRAY_MEMBER];
 }			HalfVector;
 
diff --git a/src/sparsevec.h b/src/sparsevec.h
index 6a387d4..5a88593 100644
--- a/src/sparsevec.h
+++ b/src/sparsevec.h
@@ -19,7 +19,7 @@ typedef struct SparseVector
 	int32		vl_len_;		/* varlena header (do not touch directly!) */
 	int32		dim;			/* number of dimensions */
 	int32		nnz;			/* number of non-zero elements */
-	int32		unused;
+	int32		unused;			/* reserved for future use, always zero */
 	int32		indices[FLEXIBLE_ARRAY_MEMBER];
 }			SparseVector;