Feeler gauge

A feeler gauge is a tool used to measure gap widths.

[1] They consist of a number of small lengths of steel of different thicknesses with measurements marked on each piece.

They are flexible enough that, even if they are all on the same hinge, several can be stacked together to gauge intermediate values.

[2] The lengths of steel are sometimes called leaves or blades, although they have no sharp edge.

Some feeler gauge sets have a single blade of brass due to the historical reason that early electronic ignition systems required the air gap between the reluctor and the pickup part being set with a non-ferrous metal.

Feeler gauge set (25 pieces, 0.03~1.0 mm measurement range)
Measuring the valve clearance of a DOHC engine, between camshaft and tappet (here: 0.004 in ≈ 0.10 mm ).
Tapered (upper) and parallel (lower) feeler gauges