Date: 201309
Local time of day: 0001-0600
Location: ORD.Airport, IL
Flight conditions: VMC
Light: Daylight
Aircraft: EMB ERJ 145 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground ORD
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Time Pressure; Workload; Communication Breakdown
Anomaly: Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: Taxi
Result: General None Reported / Taken
Contributing factors: Environment - Non Weather Related; Human Factors; Procedure
Primary problem: Ambiguous

Narrative: After departing our gate; we were instructed to taxi to Runway 32L at T10 via A; A7; T; T10; exactly as we had briefed the expected taxi. Passing A11; Ground instructed us to give way to a MD80 at Taxiway A9. As the jet was crossing in front of us Ground then instructed us to either: Go behind the jet on Taxiway A9 [or] follow the jet on Taxiway A9. So we turned left onto Taxiway A9. At the time; my impression was that Ground wanted us to follow the jet and then join T10 and hold short of Runway 32L. So I followed the jet on A9; then Taxiway Bravo. T10 was RIGHT there and I heard follow the jet. As we were making the right turn off Taxiway Bravo onto T10 the Ground Controller told us to 'just go ahead and turn right on T10 monitor Tower.' At this point I began to re-evaluate what the Controller had actually meant. After takeoff; the First Officer and I discussed and he said he thought they wanted us to make the left on Taxiway A9; then left on T; then T10. Based on the situation; I now believe that's what should have occurred. I'm unfamiliar with ORD; as I don't go there often. On this particular afternoon; the Ground Controller was giving many instructions with little availability/time for read-backs. Very quick change in taxi instructions with little time to digest exactly what was said. ORD mentality of 'don't stop' running through my mind. Now uncertain; but the use of the word Follow or Behind led me to think I was to follow the jet. First Officer's attention was divided on the before takeoff flow.I turned onto Taxiway Bravo; and I now believe I shouldn't have. Apparent confusion in the use of the word Follow or Behind in the clearance received. Insufficient monitor/crosscheck between myself and the First Officer. I was on a taxiway I shouldn't have been on.It's truly another lesson in CRM dynamics. Knowing how busy ORD can be; I know that my taxi speed today was appropriate; meaning the best way to give yourself more time to react is to taxi slowly. I employ this technique most always; but especially at big airports; and MOST especially ORD. As I was making the left turn onto Taxiway A9 behind the jet; I verbalized that I was 'making a U-turn' onto Taxiway B behind the jet. If the First Officer had heard/interpreted the taxi clearance correctly; I would hope that he would have corrected me at that time. That said his attention was divided on the before takeoff flow. That's my fault; as I had him start it while we were on the 'straight-away' before all the turning in our original taxi clearance. In the future; I think I will brief; or at least have the First Officer stop whatever they're doing in the event of a modified taxi instruction; I didn't stop him today and that proved consequential. In retrospect; that was the proper thing to do and the only way the First Officer would have realistically have been able to notice and correct the error; as he was obviously not really listening/comprehending what I had said.




Synopsis: An EMB-145 crew realized after the fact that having followed jet traffic to ORD Runway 32L may not have been what ORD Ground had intended.