A simultaneous submission means sending the same piece to more than one market at the same time. For poets and short-story writers it's essential — response times are long, so submitting one place at a time would mean a single piece could take years to place. But it comes with rules, and breaking them quietly damages your standing with editors.
Open SubLog — track submissions free →Don't confuse the two. A simultaneous submission is one piece sent to several different magazines at once — usually allowed. A multiple submission is sending several different pieces to the same magazine in one reading period — often not allowed, or capped. Always check each market's guidelines, because the terms aren't interchangeable and editors mean them precisely.
Most magazines accept simultaneous submissions on one condition: you must withdraw immediately everywhere else the moment a piece is accepted. Editors invest unpaid hours reading; discovering a piece they wanted is already taken elsewhere is the fastest way to sour the relationship. If a market's submission system has a "withdraw" button, use it. If not, send a short, polite email: "I'm withdrawing 'Title' from consideration, as it has been accepted elsewhere. Thank you for your time." No apology or backstory needed.
Simultaneous submitting only works if you know, instantly, everywhere a given piece is currently out. The danger isn't the strategy — it's losing the map. When an acceptance lands, you have minutes-to-hours of goodwill to withdraw cleanly, and that's impossible if you can't remember whether "The River" is at three journals or five.