Reciprocal notes

If you take any note it can be paired with another note to 'add up' to an octave.

For example if you play a G note, then a fifth above that is D and a fourth above that is G again.

Here are the pairs:

b2   7
 2  b7
b3   6
 3  b6
 4   5
b5  b5
 5   4
b6   3
 6  b3
b7   2
 7  b2

In fact you only need to learn this up to the flat 5, because above that it's the same pairs of notes—the notes are just switched.

It's also interesting that there is a symmetry to the pattern, which may help you to remember it more easily.