IB does not take the other side, it is one of the liquidity providers (either TMBR or one in the IDEAL pool). The fact that the probability is 1.0 does not mean that they take all orders; the probability is conditional on the order being executable against them. TMBR is available to non-IB customers through its quotes on NASDAQ and the ECNs where it trades. The benefits that IB customers get are: - TMBR is at NBBO for IB customers more often than for non-IB customers - when TMBR is at NBBO for non-IB customers, some of that time it is at better than NBBO price for IB customers This is largely possible because the quotes TMBR shows to IB don't suffer any latency. The "few" extra cycles it takes to make a routing decision have negligible effect on the slippage. I am now desubscribing from this thread.