A certificate of calibration traceable to NIST states that these sets have passed a test for accuracy. They’re used to check small gaps, narrow slots, and alignment. The leaves have a narrow end to measure thickness in areas where standard feeler gauges don’t fit. For example, they can measure gaps when setting engine components such as spark plugs to make sure there’s enough space so friction doesn’t lead to inefficiency.

Sets include leaves in a variety of thicknesses that are riveted together in a fan design that folds up for portability. Each leaf is marked with thickness. Leaf-securing locks hold individual gauge leaves in place.