A LIGHT TRANSPORT JET CREW TAXIING ON STL TAXIWAY F FOR A RUNWAY 12L TAKEOFF; TAXIED ONTO RUNWAY 24 BECAUSE THEY MISSED THE 12L HOLD SHORT LINE. COMPANY SOP IS FOR ONLY THE FIRST OFFICER TO HAVE THE AIRPORT CHART ON THE YOKE.

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WE WERE INSTRUCTED TO TAXI TO RWY 12L VIA TXWY F. WE EXITED THE FBO RAMP VIA TXWY INTXN F4 ONTO TXWY F AND TAXIED NW TOWARD RWY 12L. AT THE SAME TIME; AN EMBRAER REGIONAL JET TURNED L FROM TXWY P ONTO TXWY F IN FRONT OF US AND PROCEEDED NWBOUND ON TXWY F. THE EMBRAER CROSSED RWY 6/24 AND CONTINUED NW ON TXWY F. UNKNOWN TO US AT THE TIME; THE EMBRAER'S DEST WAS A FACILITY AT THE NW END OF TXWY F. SINCE WE DID NOT HEAR THE EMBRAER'S TAXI INSTRUCTIONS; WE ASSUMED WE WERE FOLLOWING THE EMBRAER TO WHAT WE THOUGHT WAS THE HOLD SHORT FOR RWY 12L. AS WE APCHED THE HOLD SHORT MARKINGS ALONG TXWY F FOR RWY 6/24; WE CAME TO A STOP AND VERIFIED THAT THERE WAS NO ARRIVING OR DEPARTING TFC ON OR APCHING EITHER END OF RWY 6/24; WHICH WAS AN INACTIVE RWY. THE CREW CONFIRMED WITH EACH OTHER THAT IT WAS GIVEN CLRNC TO TAXI TO RWY 12L AND WAS THEREFORE CLRED TO CROSS ALL INTERSECTING RWYS. WE RESUMED TAXIING NWBOUND ON TXWY F ACROSS RWY 6/24. JUST AS WE DID THIS; TWR INFORMED US THAT WE HAD ENTERED A RWY WE WERE NOT AUTH TO ENTER AND THEN ASKED US IF WE UNDERSTOOD WHAT WE JUST DID AS IN; DID WE UNDERSTAND OUR MISTAKE. WE RE-REFED THE ARPT DIAGRAM AND REALIZED THE HOLD SHORT FOR RWY 12L IS E OF RWY 6/24 AND IS FED BY TXWY S. WE INFORMED TWR THAT WE REALIZED OUR MISTAKE. TWR INSTRUCTED US TO TURN L (SW) ONTO RWY 24 AND CLRED US FOR TKOF ON RWY 12L FROM OUR TAXI POS ON RWY 24. WE MISIDENTED THE HOLD SHORT FOR RWY 12L; WHICH IS SIGNIFICANTLY STAGGERED FROM THE HOLD SHORT OF RWY 12R. THE HOLD SHORT FOR RWY 12L IS IDENTED AS A 'HOT SPOT' ON THE ARPT DIAGRAM. OUR COMPANY PROCS CALL FOR THE PNF TO MANAGE/HOLD THE ARPT CHARTS ON HIS/HER YOKE. CONSEQUENTLY; THE PF DOES NOT HAVE A SET OF CHARTS DIRECTLY IN FRONT OF HIM/HER TO EASILY AND QUICKLY REF. DURING THIS EVENT; THE PF COULD NOT EASILY IDENT THE ACTUAL HOLD SHORT FOR RWY 12L DUE TO HIS HAVING TO LOOK TOWARD THE SETTING SUN. SINCE TXWY F CONTINUES NW BEYOND ITS INTXN WITH RWY 6/24; A PLT WHO IS NOT FAMILIAR WITH STL OR DOES NOT HAVE THE ARPT DIAGRAM IN FRONT OF HIM/HER; MIGHT REASONABLY CONCLUDE THAT THE HOLD SHORT FOR RWY 12L IS BEYOND OR NW OF RWY 6/24 AND IS LOCATED AT THE NW END OF TXWY F. THAT SITUATION IS WHY THE HOLD SHORT FOR RWY 12L IS IDENTED AS A 'HOT SPOT' ON THE ARPT DIAGRAM. BASED ON THE AFOREMENTIONED CONTRIBUTING FACTORS (ARPT UNFAMILIARITY; STAGGERED RWYS; COMPANY CHART PROTOCOL AND SPECIFIC ATMOSPHERIC CONDITIONS) FOR THIS EVENT; WHAT WOULD HAVE HELPED PREVENT OUR MISIDENTING RWY 12L'S HOLD SHORT FROM TXWY F WOULD HAVE BEEN SIGNAGE; EITHER PAINTED ON OR LOCATED NEXT TO THE INTXN OF TXWY F AND S; ADVISING THAT RWY 12L BEGINS THERE. THIS WOULD BE ESPECIALLY HELPFUL DURING LOW VISIBILITY CONDITIONS AND FOR ACFT WITH FLT DECKS LOW TO THE GND. SUPPLEMENTAL INFO FROM ACN 814593: ALTHOUGH PIC QUALIFIED; DUE TO SENIORITY I WAS THE SIC AND ACTING AS THE PNF IN THE R SEAT. PER NORMAL COMPANY OPS; I HAD THE ONLY COPY OF THE ARPT DIAGRAM IN FRONT OF ME MOUNTED TO THE YOKE. AS THE PNF SIC; I FAILED TO FULLY ASSIST MY CAPT DURING THE TAXI OP SINCE I ALLOWED HIM TO ENTER RWY 6/24 WITHOUT PERMISSION. SINCE THE CAPT DID NOT HAVE A COPY OF THE ARPT DIAGRAM IN FRONT OF HIM; IT WAS MY RESPONSIBILITY TO TELL HIM HOW TO PROPERLY GET TO OUR ASSIGNED RWY. FOR THIS FAILURE; I ASSUME 60% OF THE RESPONSIBILITY. HOWEVER; THE CAPT SHOULD HAVE STOPPED THE ACFT AND NOT HAVE ENTERED ONTO RWY 6/24 UNTIL HE WAS FULLY ASSURED THAT WE WERE AUTH TO DO SO. RECOMMENDATION. THE INTXN OF TXWY F AND RWY 6/24 IS LISTED ON THE ARPT DIAGRAM AS A 'HOT SPOT.' OBVIOUSLY WE ARE NOT THE FIRST FLT CREW TO MAKE THIS MISTAKE. SO I WOULD HIGHLY RECOMMEND THE FOLLOWING: 1) STL INSTALL WARNING SIGNS AT THIS INTXN WARNING PLTS TAXIING TO RWY 12L NOT TO CROSS RWY 6/24 WITHOUT ATC AUTH. SOME FLASHING YELLOW LIGHTS WOULD ALSO HELP A LOT. 2) IT SHOULD BECOME STANDARD OPERATING PROC FOR STL GND CTLS TO WARN PLTS THAT ARE CLRED TO TAXI TO RWY 12L NOT TO CROSS RWY 6/24.