John Aniano in Central NJ
Jeremy,

I learned this from Warren years ago when I asked a similar question: the sides of firmer gouges are parallel and the sides of carving gouges taper from wide at the edge to narrow near the bolster and tang.

In addition, this seems to hold true for straight firmer and carving chisels. Thus a number 1 sweep carving chisel (flat) has tapered sides and a similar width firmer chisel would have parallel sides.

John