The light-front quantizationof quantum field theoriesprovides a useful alternative to ordinary equal-time quantization. Inparticular, it can lead to a relativistic description of bound systemsin terms of quantum-mechanical wave functions. The quantization isbased on the choice of ,where plays the role of time and the corresponding spatialcoordinate is . Here, is the ordinary time, is one Cartesian coordinate, and is the speed of light. The othertwo Cartesian coordinates, and , are untouched and often calledtransverse or perpendicular, denoted by symbols of the type . The choice of the frame of reference where the time and -axis are defined can be left unspecified in an exactlysoluble relativistic theory, but in practical calculations some choices may be more suitable than others.