Spanner or Hex fitting?
Spanner:
Here is a pedal that will only work with a 15mm spanner – the flattened part that the spanner fits onto is called, appropriately, a “spanner/wrench flat.”
Hex:
Here’s a pair of pedals – the silver one will only work with an 8mm hex wrench, while the right one will work with either a 15mm wrench or a 6mm hex wrench:
