FLC OF AN EMBRAER E120 FAILED TO HOLD SHORT OF AN ACTIVE PARALLEL RWY DUE TO A MISUNDERSTANDING OF GND CTLR INSTRUCTIONS TO FOLLOW AN ACR MD80 DURING TAXI FOR TKOF. CTLR INTERVENED AND RETURNED RPTR TO THE ORIGINAL HOLD SHORT PARALLEL RWY.

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AT SFO WE WERE CLRED TO TAXI FROM THE FBO GATES AT THE COMMUTER TERMINAL TO RWY 10R VIA TXWY B WITH THE INSTRUCTION TO HOLD SHORT AT TXWY U. WE BEGAN TO TAXI AND TO PERFORM OUR TAXI CHKLIST. PRIOR TO REACHING TXWY U; WE WERE INSTRUCTED TO FOLLOW THE ACR Y MD80 AHEAD WHICH WAS STOPPED ON TXWY B AT TXWY U. WE ACKNOWLEDGED AND READ BACK THE INSTRUCTION TO 'FOLLOW ACR Y.' THE MD80 BEGAN TO TAXI ON TXWY U ACROSS RWY 10R AND WE FOLLOWED. THE MD80 STOPPED ON TXWY U SHORT OF RWY 10L BEHIND 2 OTHER ACFT WHICH WERE WAITING TO TAKE RWY 10L TO TAKE OFF. WE ALSO STOPPED. WE WERE THEN CONTACTED BY GND CTL AND TOLD THAT WE HAD CROSSED AN ACTIVE RWY WITHOUT CLRNC TO DO SO AND TO CONTACT THE TWR. TWR INSTRUCTED US TO TURN BACK TO RWY 10R AND TAKE POS AND HOLD. WE WERE SUBSEQUENTLY CLRED FOR TKOF ON RWY 10R. WE DID WITHOUT INCIDENT. TO OUR KNOWLEDGE NO ACFT WERE TAKING OFF OR LNDG AT THE TIME OF OUR XING. THE CONFUSION AROSE BECAUSE WE ASSUMED THAT THE INSTRUCTION TO FOLLOW THE ACR MD80 SUPERSEDED THE INSTRUCTION TO TAXI TO RWY 10R. IN RETROSPECT; WE REALIZE NOW THAT IF THE CTLR HAD INTENDED US TO CHANGE TO RWY 10L HE WOULD HAVE TOLD US THAT. AS THE ACR MD80 WAS WITHIN ABOUT 100 FT OF RWY 10R WHEN WE WERE TOLD TO FOLLOW; WE ASSUMED (WRONGLY; APPARENTLY) THAT WE WERE TO FOLLOW HIM ACROSS RWY 20R. ALTERNATELY; THE CONFUSION WOULD HAVE BEEN AVOIDED IF THE GND CTLR HAD SAID; 'FOLLOW MD80 AHEAD; HOLD SHORT OF RWY 10R.' WE REALIZE THAT HE WAS NOT REQUIRED TO DO SO; BUT IT WOULD HAVE HELPED TO UNDERSTAND WHAT HE INTENDED FOR US TO DO. IF A SIMILAR SIT PRESENTS ITSELF IN THE FUTURE; WE WILL VERIFY THAT THE INSTRUCTION TO FOLLOW ANOTHER ACFT INCLUDES THE INSTRUCTION TO CROSS INTERSECTING RWYS.