diff --git a/docs/1b/ece124.md b/docs/1b/ece124.md
index 398bd02..7cf64e5 100644
--- a/docs/1b/ece124.md
+++ b/docs/1b/ece124.md
@@ -255,6 +255,17 @@ A **field-programmable gate array** (FPGA) is hardware that does not come with f
 - logic blocks (that are user-programmed to behave like gates)
     - lookup tables (LUTs) inside the logic gates, which are a small amount of memory
 
+## Gray code
+
+The Gray code is a binary number system that has any two adjacent numbers differing by **exactly one bit**. It is used to optimise the number of gates in a function.
+
+The 1-bit Gray code is $0, 1$. To convert an $n$-bit Gray code to an $n+1$-bit Gray code:
+
+- Mirror the code: $0,1,1,0$
+- Add $0$ to the original and $1$ to the new ones: $00, 01, 11, 10$
+
+Sorting truth table inputs in the order of the Gray code makes optimisation easier to do.
+
 ## VHDL
 
 VHDL is a hardware schematic language.