Doubles -- should be memorized for quick recall, sum is always even, and the pattern is counting by 2

1 + 1 = 2

2 + 2 = 4

3 + 3 = 6

4 + 4 = 8 

5 + 5 = 10

 6 + 6 =12

7 + 7 =14

8 + 8 = 16

9 + 9 = 18 

10 + 10 = 20

11 + 11 = 22 

12 + 12 = 24