Machinists Center Square, 1-1/2"

The V-groove grabbed my half-inch rod snugly, and the scribed lines crossed right at the center on the first try. Rotating it 90 degrees and marking again took maybe ten seconds total. This little square just made my layout work a whole lot less fiddly.

Used it on some round stock this morning and the V-notch seated the rod solidly, so the first scribe line was dead straight. Rotated 90 degrees as described and the cross point landed exactly where I expected. Handy little tool for the price.

The scribe lines come out perfectly crisp on 3/4" brass rod, and the 90-degree flip trick gave me a dead-on center mark in seconds. Way quicker than my old layout method.