Addition of directed numbers by laying down the "+1" and "−1" counters. Each pair of "+1" and "−1" cancel out to give "0".

[u]Reference[/u][br]Kaner, P.A. (1964) 'The addition and subtraction of negative numbers', [i]The Mathematical Gazette[/i], [b]48[/b](364), 183-185. [url=https://www.jstor.org/stable/3613541]https://www.jstor.org/stable/3613541[/url]