To ride SamTrans with Clipper card, the card must be "tagged" (read) by the Clipper card reader installed at the front of the bus near the farebox. The reader checks for a SamTrans monthly pass and local-fare credits from other agencies, computes the remaining fare and (if there is one) collects it in Clipper Cash. Note that northbound passengers on route KX to San Francisco must "tag" their Clipper card twice: once when boarding within San Mateo County (which collects a local fare or equivalent) and once before exiting in San Francisco (which collects any remaining fare).