PA31T PLT RPT ON CONFUSION REGARDING A TAXI CLRNC AND TKOF RWY ASSIGNMENT BY ATC AT HOU; TX.

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TAXI INSTRUCTIONS WERE GIVEN AT POINT 'A' TO 'TAXI TO RWY 12R VIA TXWYS Y AND E; HOLD SHORT OF RWY 12L.' I STARTED TO TAXI THE ASSIGNED RTE TILL POINT 'B.' AN AMENDED CLRNC WAS GIVEN: 'TAXI TO RWY 12R VIA TXWY H AND HOLD SHORT OF RWY 12L.' I TAXIED VIA THE NEW RTE AND STOPPED AT POINT 'C.' NEXT XMISSION: 'CROSS RWY 12L; TURN R ONTO TXWY M; HOLD SHORT OF RWY 12R; EXPECT TO DEPART RWY 12R AT INTXN D. MONITOR TWR.' AFTER A SHORT DELAY; TWR CONTACTED ME (THIS IS WHERE CONFUSION STARTS). I HEARD 'TAXI INTO POS AND HOLD RWY 12R.' THE CTLR MAY HAVE SAID OR MEANT RWY 12L. I STARTED A L TURN AND TWR IMMEDIATELY SAID; 'THAT WAS SUPPOSED TO BE A R TURN; HOLD YOUR POS THERE.' I WAS IN THE RWY AREA OF RWY 17 AT THAT TIME BUT NOT OFF RWY 12R. I STAYED IN THIS POS WHILE 2 ACFT DEPARTED IN FRONT OF ME ON RWY 12R. I WAS THEN TOLD 'CLRED FOR TKOF RWY 12R; TURN L HEADING 340 DEGS.' I WAS TOLD TO CONTACT DEP AND THE REST OF THE FLT WAS UNEVENTFUL. I MAY HAVE HAD RWY 12R IN MY HEAD SO THAT I DIDN'T NOTICE WHEN THEY ACTUALLY SAID RWY 12L. ON THE OTHER HAND; THEY MAY HAVE SAID RWY 12R AND MEANT RWY 12L. IF THE RWY CHANGE WAS MADE; IT WAS NOT EMPHASIZED. THE MISTAKE ALSO COULD HAVE BEEN MADE IN XFER FROM GND TO TWR. IT IS STILL UNCLR TO ME WHETHER THE MISTAKE WAS MADE BY ME OR ATC. NONETHELESS; NO DANGER WAS CREATED AND NO SLOWDOWNS OCCURRED. RWY 17 WAS NOT IN USE AT THE TIME.