[simulacrum]

I thought it was: There are 10 kinds of people in this world, those who understand binary, those who don't, and those who were expecting a base 3 joke.

[cgtyoder]

It really is:

There are 10 kind of people in the world: Those who know binary, those who don't, and those who start counting from zero.

[TeMPOraL]

There are 10 kinds of people in the world. Those who know binary, those who know ternary, those who know base four, ..., and those who know that every base is base 10.

[peapicker]

It really is:

There are 10 kinds of people in the world: Those who know hexadecimal, and F the rest...

[indians_pro]

>and those who start counting from zero.

That kills the joke. 'off-by-one' is subtler.