ACR FLC ACKNOWLEDGE TKOF CLRNC WHICH IS QUESTIONED AFTER BEING AIRBORNE BY ORD LCL CTLR. OTHER ACR ON INTERSECTING RWY STATES DEPARTING ACR ACKNOWLEDGED TKOF CLRNC. LCL CTLR ADVISES OF SIMILAR CALL SIGNS.

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GND METERING WAS CONTACTED ON 121.67 AND WE WERE TOLD TO MONITOR GND ON 121.75. GND CTL GAVE US TAXI INSTRUCTIONS FOR RWY 9L. WE INFORMED HIM WE WERE TOO HVY FOR RWY 9L AND REQUESTED RWY 32L/10. GND THEN ASKED IF WE WERE LIGHT ENOUGH FOR RWY 32R. WE ACCEPTED AND DOUBLECHKED THE PERFORMANCE DATA. IT WAS A SHORT TAXI AND GND TOLD US TO MONITOR TWR ON 126.9. AS WE WERE XING THE BRAVO BRIDGE; TWR TOLD US TO TAXI INTO POS AND HOLD RWY 32R. HE ALSO STATED THAT THERE WAS TFC DEPARTING ON A XING RWY. I CALLED FOR 'BELOW THE LINE' OF THE BEFORE TKOF CHKLIST; TAXIED INTO POS ON RWY 32R; AND NOTICED A JET TAKING OFF XING IN FRONT OF US. AS WE WERE COMPLETING THE CHKLIST; I HEARD 'X CLRED FOR TKOF; FLY A 090 DEG HDG.' THE FO READ BACK THE CLRNC AND WE STARTED THE TKOF ROLL. I NOTICED AN ACFT IN POS ON RWY 9L WITHOUT HIS LNDG LIGHTS ON AS WE WERE ROLLING ON RWY 32R. JUST AFTER LIFTOFF; TWR QUESTIONED ACFT Y WHY HE HAD NOT STARTED HIS TKOF ROLL. Y TOLD THE TWR THAT THE ACFT ON RWY 32R (ACFT Y) HAD READ BACK THE TKOF CLRNC. I BELIEVE THAT Y WAS THE ACFT IN POS ON RWY 9L. THERE WAS THEN A STATEMENT MADE ABOUT SIMILAR CALL SIGNS BY THE TWR. AT THIS POINT I KNEW THERE WAS SOME CONFUSION ON WHO WAS CLRED FOR TKOF. I MAINTAINED VISUAL VIGILANCE AND CONFIRMED THE 090 DEG HDG WITH THE TWR. TWR TOLD US TO FLY A 140 DEG HDG AND WERE SWITCHED TO DEP FREQ. WITHOUT HEARING THE ACTUAL TAPES; I CANNOT WRITE VERBATIM WHAT WAS SAID. OUR ENTIRE CREW (3 OF US) UNDERSTOOD WE HAD TKOF CLRNC. WE READ BACK THE CLRNC AND STARTED THE TKOF ROLL. THE ACFT ON RWY 9L (ACFT Y) DID NOT QUESTION US READING BACK THE CLRNC. THE TWR DID NOT QUESTION US AS WE READ BACK THE CLRNC. THIS IS A CASE OF VERY SIMILAR CALL SIGNS DURING THE BUSIEST TIME IN THE COCKPIT. WE WERE NOT INFORMED OF THE SIMILAR CALL SIGNS UNTIL AFTER WE WERE AIRBORNE. Y MUST HAVE BEEN GIVEN POS ON RWY 9L PRIOR TO US (ACFT X) MONITORING THE FREQ. WE ARE OFTEN INFORMED OF SIMILAR CALL SIGNS BY THE CTLR ON THE FREQ WHEN WE INITIALLY CHK ON. IN THIS CASE WE WERE NOT. X WAS READ AS XXXXXXXXX; Y WAS READ AS YYYYYYYYY. IF BOTH OF THESE QUOTATIONS ARE SAID FAST; ONE MIGHT MISINTERPRET THE NUMBER. A CORRECTIVE ACTION WOULD. BE TO READ EACH DIGIT INDIVIDUALLY. AS OUR COMPANY USES MANY 4 DIGIT NUMBERS; I TRIED THIS PRONUNCIATION OF EACH DIGIT INDIVIDUALLY 2 DAYS LATER. EACH TIME I TRIED THIS WITH DIFFERENT FACILITIES; I WAS READ BACK THE CALL SIGN IN MULTIPLE DIGITS. AGAIN; I WOULD LIKE TO REITERATE THAT I FIRMLY BELIEVE WE HAD TKOF CLRNC. THE SIMILARITY OF THE CALL SIGNS CREATED A DANGEROUS SIT THAT WAS NOT RECOGNIZED UNTIL AFTER THE EVENT OCCURRED. PRIOR KNOWLEDGE OF THE SIMILARITY AND THEN SPECIFIC PRONUNCIATION MIGHT HAVE PREVENTED THIS SIT FROM OCCURRING. SUPPLEMENTAL INFO FROM ACN 508897: ACFT WAS SHORTLY IN THE AIR AND FOLLOWED US ALL THE WAY TO FLORIDA AND THERE WAS MUCH CONFUSION BTWN THEM AND US. SEVERAL TIMES ATC REFERRED TO US BUT MEANT THEM. ON AT LEAST 1 OCCASION; OTHER ACFT RESPONDED TO ONE OF OUR ATC CLRNCS ENRTE AND WE HAD TO CLARIFY THAT THE CLRNC HAD BEEN FOR US. THIS IS ANOTHER REPEAT OF THE OLD SIMILAR CALL SIGN PROB THAT COULD BE AVOIDED.