FP is a CFD (computational fluid dynamics) method coded in Fortran for calculating the flow field and aerodynamic forces of an isolated wing or a wing-body combination in a subsonic freestream@ including the effects of shock waves. It utilises a relaxation process to solve finite-difference forms of the full nonlinear velocity-potential equation for the inviscid flow around the three-dimensional geometry. The FP method was developed over a period of years at ARA Bedford and RAE (now QinetiQ@ Farnborough) and is made available under the terms of an agreement with QinetiQ. Used originally on mainframe computers@ FP has provided valuable data in the design of a number of aircraft. With advances in computers@ it has become possible to perform runs of FP on a PC within a few minutes. This Item gives an account of the principles of FP@ together with a number of examples. Data Item 02014@ Part 2 in the Series dealing with FP@ deals with the practical aspects of running FP and its associated programs in order to obtain reliable results.