a compasses ??

A compass, or a pair of compasses -- my own maths days when we actually used those are so far away they were probably in Latin. To me a circle is just {(x,y): (x + a)^2 + (y+b)^2 = r^2}, none of this mucking around with dull sublunary approximations.

And of course the locus classicus is:

If they be two, they are two so
As stiff twin compasses are two;