  We consider successive measurements of position and momentum of a single
particle. Let P be the conditional probability to measure the momentum k with
precision dk, given a previously successful position measurement q with
precision dq. Several upper bounds for the probability P are determined. For
arbitrary, but given precision dq, dk, these bounds refer to the variation of
q, k and the state vector of the particle. A weak bound is given by the
inequality P <= dkdq/h, where h is Planck's quantum of action. It is
non-trivial for all measurements with dkdq < h. A sharper bound is obtained by
applying the Hilbert-Schmidt-norm. As our main result the least upper bound of
P is determined. All bounds are independent of the order with which the
measuring of position and momentum is made.
