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What is a spark plug spanner?

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Spark plugs are fitted and removed with spark plug spanners which are box spanners. A spark plug spanner is a box spanner designed for fitting and removing spark plugs in an engine.
Lawnmower and car engines are run by petrol or diesel engines which use spark plugs and need spark plug spanners to change them. Spark plugs are essential in every petrol or diesel engine because they provide the continuous sparking needed to light the fuel and air mixture that runs the engine.
Spark plug spanner with a built in swivel tommy bar. Spark plug spanners may have a removable tommy bar (see What is a box spanner?) or they may have a built-in tommy bar which can be swivelled into different positions to turn the spanner.
Spark plug spanner covers spark plug to turn it. The spanners are deep enough to fit over the protruding end of the spark plug.
Spark plug spanner sizes and hexagonal profile. They have hexagonal profiles and come in a range of sizes from about 10mm (3/8″) to 21mm (13/16″). Each engine requires a specific size of spark plug so the spanner needs to be purchased to fit it.
Older vehicles will use imperial measurements of spark plugs and spanners. The majority of engines use metric spark plug sizes but older machinery (usually pre-1970s) will use imperial sizes. This is worth remembering when measuring the size of spark plug spanner required.