BE40 CREW WAS CLRED TO TAXI TO RWY 21L. THE TWR CTLR SAID THEY SHOULD NOT HAVE CROSSED RWY 25 AT LUK.

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GND CTL INSTRUCTIONS WERE TO TAXI FROM FBO VIA TXWY A TO RWY 21L. WE TAXIED TO A POS BTWN RWY 25 APCH END AND RWY 21L APCH END AND STOPPED. GND CTL CALLED TO SAY WE SHOULD HAVE STOPPED PRIOR TO XING RWY 25. BEING CAPT AND ON THE RADIOS; I APOLOGIZED FOR THE ERROR; BUT AM UNCLR ABOUT POS OF HOLD LINE FOR RWY 21L. SUPPLEMENTAL INFO FROM ACN 624436: MY CAPT AND I; BOTH OF US BEING TYPE RATED IN THE ACFT; AND MYSELF FLYING L SEAT ON THIS LEG) WERE GIVEN A TAXI CLRNC TO TAXI FROM THE FBO RAMP AT LUK TO RWY 21. IN ORDER TO 'TAXI TO RWY 21;' WE THOUGHT THAT WE HAD TO CROSS RWY 25 AT THE THRESHOLD TO HOLD SHORT RWY 21. AFTER XING THE HOLD SHORT LINE FOR RWY 25 IN ORDER TO HOLD SHORT FOR RWY 21; WE WERE TOLD BY GND/TWR THAT WE SHOULD HAVE HELD SHORT FOR RWY 25. I BELIEVE THAT A CLOSER LOOK AT THE ARPT DIAGRAM WOULD HAVE CONFIRMED FOR US WHERE WE WERE TO HOLD SHORT; OR WOULD HAVE PROMPTED CLARIFICATION FROM THE TWR IF THERE WAS ANY QUESTION. I FEEL THAT A CLOSER LOOK AT THE ARPT DIAGRAMS ON OUR (THE CREW'S ) BEHALF WILL PREVENT THIS FROM HAPPENING AGAIN.