[skc]

Silly question, why does the author only approximate the number of possible positions in chess?

[tromp]

It's not like Go where you can actually count the number of reachable positions.

Deciding whether a single chess position is reachable can be really hard. There is a whole class of so called "retrograde" chess problems focussed on that.

For example, is the following position reachable?

White: Kc3 Ba4 Black: Kd1 Rb5 Bd5

[joelthelion]

Because the real number is unknown?