A pastry blender or pastry cutter is a device used to cut a solid fat (such as butter, margarine, lard, or shortening) evenly into a dry, usually flour-based, mixture. A pastry cutter will typically have four or five blades or stiff wires running parallel, which curve into a handle on both ends creating a "D" shape.
The purpose of a pastry blender is to distribute the fat into the mixture without combining them. When a dough made with this technique is rolled flat, these small pieces of fat are extended to discs in different levels of the dough. When this dough is cooked, the fat forms small pockets which give pastry its delicate and flaky texture.
A pair of butter knives is typically an acceptable substitute for a pastry cutter, though the process may be slower.