The locus of a point P which moves on one side only of a straight line XY so that ∠XPY = 90o is a semicircle. This is because the definition of a semicircle is the set of all points on a plane that are equidistant from a fixed point (the center) and lie on one side of a fixed line (the diameter). In this case, the fixed line is XY, and the fixed point is the midpoint of XY.