3D printers are defined by how they function. Most FFF 3D Printers are called Cartesian. This means that they build 3D models in a rectilinear build volume by moving a filament extruder in X (left to right) and Y (front to back) axes to extrude filament in the shape of the part being built. They do this one layer at a time, moving the extruder Up, or the build plate Down, vertically in the Z axis. Some 3D printers only move the extruder in one axis (X), and the build plate in the other (Y).