BTO: buy to open. STO: sell to open. BTC: buy to close a Short. STC: sell to close a Long.
What these codes are
The four codes are every action in options trading: BTO (Buy To Open) buys to open a new right. STO (Sell To Open) sells to open a new obligation. BTC (Buy To Close) buys back to end an existing obligation. STC (Sell To Close) sells to end an existing right. Direction alone is not enough. Opening versus closing is half the meaning.
Why the distinction matters
“Selling a call” is an ambiguous phrase: STO is a bearish bet with a new obligation, while STC just takes profit on a right you own. They are opposites separated by one word. The broker needs the intent to set margin and requirements, and the market needs it to count Open Interest (two opens create a contract, and two closes erase it).
Life cycle
Every position opens with one of the two opening actions and closes with its counterpart: BTO is closed by STC, and STO is closed by BTC (or both end by exercise, assignment or expiring worthless). Reading a flow screen without both dimensions mixes up traders coming in with traders going out.
Hypothetical example
A flow screen shows ten thousand calls sold. If it was STO, it is a new institutional bearish bet. If it was STC, it is a fund taking profit on an old bullish one. The two readings are opposite and the line is the same. (Open interest the next day usually settles it.)
Common mistake
Sending an opening action when you meant to close: if you hold a contract and send STO instead of STC, you end up with two opposing open positions and margins you never intended. Read the middle word twice before you send.
Why the market needs this distinction
Your broker needs the intent to set margin and requirements, and the market needs it to count open interest: two opens create a contract and two closes erase it. Send an opening order when you meant to close, and you end up with two opposite positions and margin you never intended.
Where to see it on the platform
The trading journal records every trade with its four-part action, and the learning pages walk through the full cycle with examples.
The trading journal records every trade with its four-part action, so you can read the full life cycle.
