Feeler gauge
A feeler gauge is a simple tool used to measure thicknesses. Feeler gauges are mostly used in engineering to measuring the clearance between two parts.
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. It's common to have two sets for imperial units and metric measurements although the pictured set has both measurements recorded on each blade.
A similar device with wires of specific diameter instead of flat blades is used to set the gap in spark plugs to the correct size; this is done by increasing or decreasing the gap until the gauge of the correct size just fits inside the gap.
The lengths of steel are sometimes called blades, although they have no sharp edge.