17 C.F.R. § 242.610 : Commodity and Securities Exchanges — SECURITIES AND EXCHANGE COMMISSION — REGULATIONS M, SHO, ATS, AC, NMS, AND SBSR AND CUSTOMER MARGIN REQUIREMENTS FOR SECURITY FUTURES —REGULATION OF THE NATIONAL MARKET SYSTEM — Access to quotations

(a) Quotations of SRO trading facility. A national securities exchange or national securities association shall not impose unfairly discriminatory terms that prevent or inhibit any person from obtaining efficient access through a member of the national securities exchange or national securities association to the quotations in an NMS stock displayed through its SRO trading facility.
(b) Quotations of SRO display-only facility .
(1) Any trading center that displays quotations in an NMS stock through an SRO display-only facility shall provide a level and cost of access to such quotations that is substantially equivalent to the level and cost of access to quotations displayed by SRO trading facilities in that stock.
(2) Any trading center that displays quotations in an NMS stock through an SRO display-only facility shall not impose unfairly discriminatory terms that prevent or inhibit any person from obtaining efficient access to such quotations through a member, subscriber, or customer of the trading center.
(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:
(1) If the price of a protected quotation or other quotation is $1.00 or more, the fee or fees cannot exceed or accumulate to more than $0.003 per share; or
(2) If the price of a protected quotation or other quotation is less than $1.00, the fee or fees cannot exceed or accumulate to more than 0.3% of the quotation price per share.
(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:
(i) Displaying quotations that lock or cross any protected quotation in an NMS stock; and
(ii) Displaying manual quotations that lock or cross any quotation in an NMS stock disseminated pursuant to an effective national market system plan;
(2) Are reasonably designed to assure the reconciliation of locked or crossed quotations in an NMS stock; and
(3) Prohibit its members from engaging in a pattern or practice of displaying quotations that lock or cross any protected quotation in an NMS stock, or of displaying manual quotations that lock or cross any quotation in an NMS stock disseminated pursuant to an effective national market system plan, other than displaying quotations that lock or cross any protected or other quotation as permitted by an exception contained in its rules established pursuant to paragraph (d)(1) of this section.
(e) Exemptions. The Commission, by order, may exempt from the provisions of this section, either unconditionally or on specified terms and conditions, any person, security, quotations, orders, or fees, or any class or classes of persons, securities, quotations, orders, or fees, if the Commission determines that such exemption is necessary or appropriate in the public interest, and is consistent with the protection of investors.

17 C.F.R. §242.610