Originally Posted by Sheldon Brown
It's a funny thing...back when freewheels were the norm, it was common to do custom ratios for customers, and all good mechanics were adept at it. Now we use cassettes, and hardly anybody customizes them any more. Most folks assume that it is harder to customize cassettes than freewheels, but the opposite is actually true!
On the other hand, we used to only have 5 or 6 cogs so you had to make the best use of what you had. Today, with 9 or 10 cogs on a cassette, there's less need to customize. You might like to have a ratio to fit between your 12 and 13 tooth cogs, but you can't very well make a 12 1/2 tooth cog.