What is a Leg?
In the brokerage world, a leg is an individual component of a multistep trade.
How Does a Leg Work?
Why Does a Leg Matter?
Timing is everything is multistep trades. Accordingly, in John's case, his broker should try to execute both legs simultaneously, or at least very close in time, in order to avoid the risk of price changes for those products as the trading day progresses.