diff --git a/HARDWARE.md b/HARDWARE.md
index 258389a..f0ec56d 100644
--- a/HARDWARE.md
+++ b/HARDWARE.md
@@ -39,13 +39,16 @@ Unknowns are marked `TBD` instead of guessed.
 - Raspberry Pi drives each relay channel independently.
 - Thermocouple boards are connected to Pi GPIO.
 
-## GPIO Wiring (User-Confirmed)
-- Thermocouple channel 1 GPIO lines: `GPIO 5`, `GPIO 8`, `GPIO 7`.
-- Thermocouple channel 2 GPIO lines: `GPIO 11`, `GPIO 12`, `GPIO 13`.
-- Channel 2 thermocouple mapping corresponds line-for-line to channel 1 mapping (`5/8/7` -> `11/12/13`).
-- Relay control (channel 2): `GPIO 9`.
+## GPIO Wiring (Clarified)
+- Thermocouple channel 1 (as originally noted): `GPIO 5`, `GPIO 8`, `GPIO 7` (BCM numbering).
+- Thermocouple channel 2 was written as `11,12,13`; this has been clarified as **physical header pins**.
+- Thermocouple channel 2 validated mapping:
+  - Physical pins `11,12,13` => BCM `17,18,27`.
+  - Read test on BCM `17,18,27` returns valid MAX6675 words with open-thermocouple fault bit set when probe is disconnected (expected).
+- Alternate interpretation (BCM `11,12,13`) returned all-zero frames and appears incorrect for channel 2.
+- Relay control (channel 2): `GPIO 9` (BCM).
 - `TBD`: confirm relay control GPIO for channel 1 in this dual-channel prototype wiring doc.
-- Note: these dual-channel GPIO assignments differ from the current single-channel `piNail2/config.json` runtime pins.
+- Note: these dual-channel assignments differ from the current single-channel `piNail2/config.json` runtime pins.
 
 ## Monitor Subsystem
 - Separate 12V power supply for the built-in monitor.