Für beliebige reelle Zahlen x, y gilt
x + y = y + x
Summanden sind also vertauschbar:
42 + 13 = (+42) + (+13) = (+13) + (+42) = 13 + 42
-42 + 13 = 13 + (-42) = 13 – 42
42 – 13 = 42 + (-13) = (-13) + 42 = -13 + 42
-42 – 13 = (-42) + (-13) = (-13) + (-42) = -13 – 42