  We prove an uncertainty relation, which imposes a bound on any joint
measurement of position and momentum. It is of the form $(\Delta P)(\Delta
Q)\geq C\hbar$, where the `uncertainties' quantify the difference between the
marginals of the joint measurement and the corresponding ideal observable.
Applied to an approximate position measurement followed by a momentum
measurement, the uncertainties become the precision $\Delta Q$ of the position
measurement, and the perturbation $\Delta P$ of the conjugate variable
introduced by such a measurement.
  We also determine the best constant $C$, which is attained for a unique phase
space covariant measurement.
