Lathe Size?(Measure Lathe Swing)

Lathes are advertised with a 12 inch swing over the bed or 6 inch swing over the bed or even 20 inches over the bed(some advise the distance over the banjo is smaller by at least 31/2 inches therefore a 12 inch bowl may only measure 91/2 inches. My understand is that you measure from the head stock to the bed and double that measure which is how large a bowl you can turn. Example -12 inches does that mean you can turn a bowl 24 inches? Someone please clear this up for me