The beam tagging silicon (or ``tagging'') measures Ee, the incident electron energy. The tagging consists of five planes of silicon strip detectors, two on either side of the momentum recombining dipoles and one positioned between the two dipole magnets. This arrangement is shown in beam:beamline. Each plane has 256 strips with a pitch of 300 . Since two dimensional position information is not needed for measuring momentum, the tagging only measures position in the bend (horizontal) view. The total active area of each plane is 7.7 cm wide by 5.7 cm high. The electron tagging system used in E687 is described in Alimonti:1992kv; the design and operation of the system was unchanged for FOCUS. The momentum resolution of the beam tagging system is about 2%.