A Fine Measurement

I shifted my stance from the magnifying lens and checked the perimeter along the window frame. Resting against the corner was a wooden folding ruler, its brass-hinged segments folded into a tight zigzag. The first joint was stiff. I braced my boots against the sill, gripped the brass pivot with both gauntlets, and applied calculated leverage. It snapped open with a crisp, dry pop, extending its ruled black increments across the timber. I laid my straightened fastener parallel to the millimeter notches to gauge its length. Precisely forty millimeters. Reliable data. I folded the joint flush against the casing and resumed my post.