The second number is the pitch and relates to the distance traveled.

The first number is the diameter and is required to asob power. Actually it can get more complicated than that but that's one way of thinking of it.

On a typical .46 engine you can run a 11-6 on it. but you can also run a 10-7 or a 9-8 or a 12-5.

The larger diameter - lower pitch prop will give you more thrust and would be better on an airplane that you didn't want to fly very fast but have good vertical ability. (A 3D Fun Fly plane).

The smaller diameter prop with the higher pitch will give you more speed in a small streamlined airplane.

On the 40 - 90 size 2 strokes, the 6" pitch is usually considered normal.

On the bigger engines, you go to a 8" pitch to be normal.

enjoy,

Jim