On Thu, 16 Jul 2009 22:16:15 +0100, had this to say:

On 16 Jul,
John wrote:

Isn't M6 similar to 2BA?

)BA is is interchangable with M6 in one direction. I can't remember if it's
nuts (just about) fit bolts or vice versa.

Contrary to popular belief, BA sizes were originally specified in metric.


BA is based on the metric system, apparently.

I don't know how the diameters evolved, but the thread pitch was based
on powers of 0·9mm, so that say, 0 BA is 0·9^0 (1mm), 1 BA is 0·9^1
(0·9mm).... 6 BA is 0·9^6 (0·531441mm), and so on.

I think this was rounded to the nearest 'mil' (1/1000 inch) in proper
English practice.

--
Frank Erskine