Marking Gauge

A marking gauge scribes a fine line into wood at a fixed distance from a reference edge, set by sliding an adjustable fence along the tool's beam and locking it in place. Dragging the gauge's pin or small blade along the workpiece with the fence riding the edge produces a line that's both dead straight and exactly repeatable across multiple pieces -- something a pencil and rule can't reliably match, since a pencil line has real width and human measurement has real slop.

A cutting gauge (a small blade instead of a pin) severs wood fibers cleanly ahead of a saw or chisel, reducing tearout on the finished cut -- useful for laying out dovetails and tenon shoulders where a crisp line matters more than it does for rough layout.