diff --git a/lib/std/Io.zig b/lib/std/Io.zig
index 9bd7ec5a0dc76ed8628b7a9933b7c1bf41203ec3..3924d8ca00f44db50cec2301add1fedf7573195c 100644
--- a/lib/std/Io.zig
+++ b/lib/std/Io.zig
@@ -1740,9 +1740,11 @@ pub fn futexWake(io: Io, comptime T: type, ptr: *align(@alignOf(u32)) const T, m
 /// Mutex is a synchronization primitive which enforces atomic access to a
 /// shared region of code known as the "critical section".
 ///
-/// Mutex is an extern struct so that it may be used as a field inside another
-/// extern struct.
-pub const Mutex = extern struct {
+/// Mutex is not an extern struct because locking and unlocking go through the
+/// futex calls of `Io`, which are not required to work across processes or
+/// compilation units. Code that needs a fixed layout can build its own lock on
+/// a `u32` with the same futex calls.
+pub const Mutex = struct {
     state: std.atomic.Value(State),
 
     pub const init: Mutex = .{ .state = .init(.unlocked) };