diff --git a/Appendix-C:-Firmware-programming.md b/Appendix-C:-Firmware-programming.md
index cf0d057..b15cc93 100644
--- a/Appendix-C:-Firmware-programming.md
+++ b/Appendix-C:-Firmware-programming.md
@@ -18,7 +18,7 @@ The Arduino Nano offers a very simple and easy way of programming the microcontr
 The base PCB has a 6-pin ICSP header. The pins are numbered like this:
 
 ```
- 1 -> ?  o <- 2
+ 1 -> ■  o <- 2
  3 -> o  o <- 4
  5 -> o  o <- 6
 ```