Addition of Directed Numbers - Counters Model
Addition of directed numbers by laying down the "+1" and "−1" counters. Each pair of "+1" and "−1" cancel out to give "0".
Reference
Kaner, P.A. (1964) 'The addition and subtraction of negative numbers', The Mathematical Gazette, 48(364), 183-185. https://www.jstor.org/stable/3613541