Why did my Buy Limit order get filled during pre-market hours?

There are two reasons why an order would be filled during the pre-market session:

  1. The submitted order was set to work outside regular trading hours (RTH) or pre-market hours.
  2. The order was for an OTC/Penny stock which does not have a pre-market session but starts regular trading hours at 8:00 EST.
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Why did my order not fill overnight?
What are the key considerations for trading outside Regular Trading Hours (RTH)?
Will my order that is set to work outside RTH work in the US overnight trading session?
How do I trade during pre-market hours?