17 C.F.R. § 242.610
(b) Quotations of SRO display-only facility.
(c) Fees for access to quotations. A trading center shall not impose, nor permit to be imposed, any fee or fees for the execution of an order against a protected quotation of the trading center or against any other quotation of the trading center that is the best bid or best offer of a national securities exchange, the best bid or best offer of The Nasdaq Stock Market, Inc., or the best bid or best offer of a national securities association other than the best bid or best offer of The Nasdaq Stock Market, Inc. in an NMS stock that exceed or accumulate to more than the following limits:
(d) Locking or crossing quotations. Each national securities exchange and national securities association shall establish, maintain, and enforce written rules that:
(1) Require its members reasonably to avoid: