A feeler gauge is a tool

Tool

A tool is a device that can be used to produce an item or achieve a task, but that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such...

 used to measure gap widths. Feeler gauges are mostly used in engineering

Engineering

Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...

 to measure 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 is common to have two sets for imperial unit

Imperial unit

The system of imperial units or the imperial system is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The system came into official use across the British Empire...

s (typically measured in thousandths of an inch) and metric

Metric system

The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...

 (typically measured in hundredths of a millimetre

Millimetre

The millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length....

) measurements.

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.

Taper feeler gauge

A taper feeler gauge is a feeler gauge of tapered, as opposed to parallel, shape. The blade of the gauge is of a constant thickness, and the two types of gauge are used in a similar way.