We consider a continuous time control system. The control involvs the state trajectory jump discontinuities in n interior times, called impulse moments. We deal with three kind of controls: (1°) The impuls moments are fixed. The parameters describing the jump are decision variables; (2°) Jump parameters are fixed. The moments are decision variables; (3°) Both the moments and the parameters are decision variables. In each case a maximum principle is stated. The paper shows that the proofs of the principle for cases (2°) and (3°) may be reduced to (1°). A simple proof in case (1°) is given.