Measuring recurve bow length.

[BAK]

LBR nailed it.  String length is determined by what the correct brace height is when strung.  We use these other methods to approximate that.

[Orion]

"AMO bow length is 3 inches longer than the string master that properly braces the bow," is really saying the same thing as the more informal statement that the AMO standard is a string that's three inches shorter than the AMO bow length. It's assumed that that string length will give the proper brace height. Just the same statement in reverse.

The problem arises, as Chad has pointed out, in that the measurement hasn't been around for ever, and, more importantly, it is not universally followed.