diff --git a/cranelift/frontend/src/variable.rs b/cranelift/frontend/src/variable.rs
index e96fe7d949..f83c5ad6a3 100644
--- a/cranelift/frontend/src/variable.rs
+++ b/cranelift/frontend/src/variable.rs
@@ -1,7 +1,12 @@
 //! A basic `Variable` implementation.
-//! 
+//!
 //! Frontends can use any indexing scheme they see fit and
 //! generate the appropriate `Variable` instances.
+//!
+//! Note: The `Variable` is used by Cranelift to index an array containing
+//! information about your mutable variables. Thus, when you create a new
+//! `Variable` you should make sure that the index is provided by a counter
+//! incremented by 1 each time you encounter a new mutable variable.
 
 use core::u32;
 use cranelift_codegen::entity::EntityRef;