Logic gate used in reversible computation. Has 2 inputs:

• Input A is a control line, and is copied unchanged to output A'.
• Input B is the data line; it is copied unchanged to output B' if A=0, otherwise it is inverted on B'.

3 controlled not gates can be used to exchange a pair of lines (useful if you're also working in 2 dimensions).

Feynman notation:

A ---o---
     |
B ---x---

Abbreviation: CN.