> Is spanner the Brit name for wrench, like open end or box wrench
> or perhaps crescent wrench?

Both terms get used, but are usage dependant. A spanner is always used on a 
nut, or similar formed point designed for a tool to lock onto.

A wrench will typically have claw teeth on the jaws for gripping on an 
unformed surface, like a pipe wrench onto round pipe. However a wrench may 
still be used on formed nuts, as in a well wrench for removing the nuts 
holding a car wheel.

Perhaps this derives from the verb 'wrench' as in 'to wrench something 
round' whereas a spanner is always regarded as being used in a controlled 
manner (until it slips off the nut and bangs your hand against the exhaust 
pipe ...). 

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