Feeler gauges are thin strips of metal or plastic that are inserted into gaps and used to measure the gap width. Also known as thickness gauges, these tools have straight sides or sides that taper inward toward the end of the piece. These gauges are marked with the thickness graduations and often the standard and metric equivalent so the thickness can be read without needing another gauge to measure it. They are commonly used by engineers and machinists for heavy and light production, woodworking, and automotive applications.