fact family - a group of related addition and subtraction facts.
Ex.: 2 + 4 = 6
4 + 2 = 6
6 - 2 = 4
6 - 4 = 2

related - addition facts and subtraction facts are related if they have the SAME numbers.
Ex.: 2 + 3 = 5 and 5 - 2 = 3 are related.