Wall Street's top regulator on Wednesday unveiled rules allowing stock markets to price shares in increments of half a penny, rather than the current minimum size of 1 cent, aiming to promote more competitive pricing and reduce investor costs.

That might help trading venues such as the New York Stock Exchange and Nasdaq Inc., among others, to better compete with wholesalers that can quote in finer increments outside of an exchange.

The proposals being considered by the Securities and Exchange Commission are tamer versions of the ambitious revamp of stock trading that Chair Gary Gensler proposed two years ago.