An Explanation

Numbers that can be divided by only themselves and 1 are considered prime numbers. Most numbers can be divided by more numbers evenly. These even divisors are an indication of a number's usefulness in grid architecture. By looking at the even divisors of certain numbers, we can see that small number sets will rarely give us all four sets of halves, thirds, fourths, and fifths.

10 gives us fifths and halves. 12 gives us thirds, fourths and halves, and so is most often used in a columning system. 15 can give us thirds and fifths but not halves, and 20 can give use fourths, fifths and halves, while 30 can give us thirds, fifths and halves. 60 Would be the first number that gives us all four of those divisions.

Basically the more even divisors you can have, the more utilitarian your grid is. But the more total columns you have, the more unwieldy it becomes. There is a sweet spot for every design.