In electrostatic quadrupoles, which are used to focus state−selected beams of polar molecules, it is customary to use a beam stop to prevent the direct nonfocused beam from striking the detector. An alternative method would be the use of a pair of electrostatic quadrupoles with a central collimating orifice, as described below, to separate the state−selected beam from the parent beam. It is shown that, in general, a double quadrupole system with a central orifice has a much lower background signal and can have a larger transmission than the quadrupole system using a beam stop. The arguments given are for a lens system containing no field−free regions; the extension to a system with field−free regions is straightforward.