A sell short order for any listed (exchange traded) security can be executed
only on an uptick or zero-plus tick. This means that for a short sell order to be
executed, the price of the last transaction on the primary exchange must be
higher than the price of the previous transaction, or equal to the price of the
last higher transaction. For example, if trades are executed sequentially at
$51, $52, and $52, a short sale may be executed on the first trade at $52 (the
uptick) or the second trade at $52 (a zero-plus tick). These rules are
designed to keep investors from shorting a stock that is already falling
thereby accelerating its downward spiral even more.

For Nasdaq stocks, a short sale will be executed only if the trade can be
done at a price higher than the previous different price.