The SDLC adheres to important phases that are essential for developers—such as planning, analysis, design, and implementation—and are explained in the section below. This includes evaluation of the currently used system, information gathering, feasibility studies, and request approval. A number of SDLC models have been created, including waterfall, fountain, spiral, build and fix, rapid prototyping, incremental, synchronize, and stabilize. The oldest of these, and the best known, is the waterfall model, a sequence of stages in which the output of each stage becomes the input for the next. These stages can be characterized and divided up in different ways, including the following: