A Marking Gauge is a woodworking tool used to scribe a line parallel to the edge of a board. It consists of a stock, a fence, and a marking point, which is typically a sharpened metal rod. The stock is used to hold the fence in place
From Wikipedia
A marking gauge, also known as a scratch gauge, is used in woodworking and metalworking to mark out lines for cutting or other operations. The purpose of the gauge is to scribe a line parallel to a reference edge or surface. It is used in joinery and sheetmetal operations.