Re: If I have both a combo and a double square...
William Duffield, on the Cohansey
>I have a 6" rule with a combo head and a center finder head, 12" and 24" rules with combo, center finder and protractor heads, and a 4" double square. All rules are in 8ths, 16ths, 32nds and 64ths. I use them all, but the one that I use the most is the 4" double square. It fits in my shirt pocket, while the 6" doesn't, and there is still room for pens, sharpies and a cellphone. A combo head (even a 4" one) is much clumsier to carry in a pocket because it tends to catch going in and coming out.
Four different measurement scales are important, as I find the 64th marks get in the way when I only need to measure 8ths or 16ths.The double head also is a lot easier to set to and read a particular measurement, as it is much less often necessary to remove the blade to turn it around or turn it over to position the right scale against the side of the head than a combo head.
I haven't bothered to get a center finder head for the 4" because I find the 6" convenient enough for most of my lathe work.
I also have another very useful gizmo (a Lufkin equivalent to Starrett No. 289 series) that will hold two different rules perpendicular to each other.
I would suggest you keep both of the squares you have, and consider adding more as they become available in the future. Prices are often quite reasonable on the used tool market.