Hard-edge is a painting style that uses very straight and clean linear patterns and/or lines to create a 3-D effect on a 2-D surface, hence a canvas. Many tools can be used to do such work. Most often, normal tape can be used to make clean lines. Using a flat and very soft paintbrush can have a nice smooth look without seeing any of the marks usually left by rough bristles. Palette knives can also be used to create these patters.
See also: abstract art