diff --git a/README.md b/README.md
index 3d643d2..744cf12 100644
--- a/README.md
+++ b/README.md
@@ -27,4 +27,23 @@ character set, along with these messages:
 
 Any unrecognized word or symbol outside the 32-129 range is ignored.
 
+The Decatron tubes respond to 0-9 and "off", same as the others, along with
+the following actions:
+
+* "inc" will increment the tube's current number value.
+  If the value overflows from 9 back to 0, the tube will generate a "carry"
+  message.
+* "dec" will decrement the current value.
+  If the value wraps from 0 back to 9, the tube will send out a "borrow"
+  message.
+* "get" will query the current state of the tube, responding with a single
+  digit 0-9 or the word "off".
+
 Tubes emit a small amount of light when displaying something.
+
+Nixies can only be mounted on the floor, while Decatrons can be mounted on a
+wall (or a ceiling if so desired).
+
+A Decatron has a small grey spot on its internal insulator to mark the "0"
+position.
+