What does on center mean?

Dual bananas are made so that you have to drill holes with a 0.75" distance (about 19 mm). It is easy to check if you hold it in your hands.
It is standard so there are many plugs with such a distance. Even Europe mains plugs (Shuko, CEE7 etc) are made with the same distance.
 

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It's pretty simple, when you mark a hole to be drilled you typically make scribe marks horizontally and vertically and use a center punch to provide a registration for the drill bit. The center punch marks the center of the hole and for these sockets the specified spacing is 0.75" on the center punch marks or on center for shorthand.