WDB TAXIES ONTO WRONG TXWY.

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ON TAXI OUT OF SPOT XX; GND CTL GAVE US TAXI INSTRUCTIONS OF 'TAXI VIA Q; C; TO RWY 18L.' I READ BACK THOSE INSTRUCTIONS VERBATIM; AND SINCE THIS WAS A CHANGE OF RWY FROM WHAT WE EXPECTED; I BEGAN MAKING CHANGES TO THE FMS; ETC. THE CAPT CALLED TO CONFIGURE THE ACFT AND RUN THE CHKLIST AND FINISH MAKING THE CHANGES REQUIRED FOR THE RWY CHANGE. WHILE I WAS MAKING THESE CHANGES THE CAPT TURNED ONTO TXWY Z WHICH WAS NOT PART OF OUR TAXI INSTRUCTIONS. IMMEDIATELY AFTER THE TURN I NOTICED THIS AND SAID TO THE CAPT THAT WE WERE CLRED VIA TXWY Q; NOT TXWY Z. IMMEDIATELY AFTER I SAID THIS; GND CTL CALLED TO SAY THAT THEY HAD WANTED US TO TAXI ON TXWY Q. THE CAPT THEN RESPONDED TO GND CTL THAT HE HAD NOT UNDERSTOOD THE TAXI INSTRUCTIONS AND COULD THEY SPEAK MORE CLRLY NEXT TIME. THIS WAS THE FIRST THAT I LEARNED THAT THE CAPT HAD NOT UNDERSTOOD THE TAXI INSTRUCTIONS. GND CTL'S INSTRUCTIONS HAD BEEN CLR TO ME; AND IF THE CAPT WAS NOT SURE; THEN HE COULD HAVE ASKED ME OR REQUESTED THAT GND REPEAT THE INSTRUCTIONS.