This excerpt taken from the NDAQ DEF 14A filed Apr 21, 2006.
Series D Preferred Stock
The SEC requires that NASD retain greater than 50% of the voting control over us. The one outstanding share of Series D stock issued to NASD ensures that NASD maintains voting control until we meet SEC conditions to operate as an exchange and no longer rely on the NASDs SRO license. The voting power of the share of Series D stock is recalculated for each matter presented to stockholders. NASD is entitled to cast the number of votes that, together with all other votes that NASD is entitled to vote by virtue of ownership, proxies or voting trusts, enables NASD to cast one vote more than one-half of all votes entitled to be cast by holders of Nasdaq securities. Once we become operational as an exchange, the share of Series D stock will automatically lose its voting rights and will be redeemed by us for $1.00.