There are three sizes of plug in normal use, 10mm, 14mm and 18mm. (They are identified by their thread size)
The socket sizes for these are 5/8" (16mm) 13/16" (21mm) and 15/16" (24mm).
However, many 14mm plugs (the most common size) have 5/8 hexagons, and many 18mm plugs have 13/16 hexagons!
hence why a 10mm spark plug socket fits 🙂
