Something that is oblique is slanted or not perpendicular. Oblique coordinates are measured on a plane coordinate system whose axes are not perpendicular. An oblique angle is any angle except a right angle and an oblique triangle is any triangle that doesn't contain a right angle (i.e., one in which one of the internal angles is obtuse). The first reference to 'oblique-angled triangles' may have been by Thomas Blundevil in his Exercises in 1594.