diff --git a/doc/7-systems/pokey.md b/doc/7-systems/pokey.md
index 1125da1a..cd94652f 100644
--- a/doc/7-systems/pokey.md
+++ b/doc/7-systems/pokey.md
@@ -32,3 +32,6 @@ a sound and input chip developed by Atari for their 8-bit computers (Atari 400,
     - filtered output on channel 2 (I suggest you to set channel 4 volume to 0).
     - use for PWM effects (not automatic!).
   - bit 0: 15KHz mode.
+- **`12xx`**: toggle two-tone mode.
+  - when enabled, channel 2 modulates channel 1. I don't know how, but it does.
+  - only on ASAP core.