diff --git a/operators/cuda/rotary.h b/operators/cuda/rotary.h
index a365c2470..b5e27b877 100644
--- a/operators/cuda/rotary.h
+++ b/operators/cuda/rotary.h
@@ -8,6 +8,16 @@
 
 namespace contrib {
 
+/**
+*  Y = Rotary(X) is equivalent to if side == LEFT:
+*
+*   N = X.shape[-1]
+*   Y = X.copy()
+*   Y[...,:N/2] = X[...,N/2:]
+*   Y[...,N/2:] = -X[...,:N/2]
+*
+*   And the opposite if side == RIGHT.
+*/
 template <typename T>
 struct Rotary {
   template <typename TDict>