What's the difference?
This was one of my first newbie questions, and for a while - even after I knew the answer - I was still confused. Call me slow, but the concept didn't really sink in until I started submitting manuscripts in earnest. So, in case you are also confused, here's the low-down:
This means that you are submitting more than one story (poem, novel, whatever) to the same market at the same time. Or that you are submitting a work to a magazine (publishing company, agent) when you already have one waiting in their slush pile. So let's say, for example, that you've mailed your short story, The Flight of the Walrus, to the magazine The Walrus Review. If you turn around the next day and mail a second story to this same market, then you are submitting multiple stories.
Generally, most markets frown on multiple submissions (although poetry is sometimes an exception). Read the submission guidelines carefully before sending in your work. Some markets even have a policy that states an author should wait a certain length of time between submissions.
This takes place when you send a single story (novel, poem, whatever) to more than one market. So, using the previous example, you send your short story, The Flight of the Walrus, not just to The Walrus Review but also to Walrus Weekly and Walrus World.
Most of the time it is okay to send your work out to more than one market. It certainly can save time! But it is considered proper etiquette to let the markets know that you are doing this. You needn't tell them precisely where you are sending your work, but at least mention it in your query letter (something along the lines of, "I'm considering other markets" or "I have submitted this piece to other magazines as well as yours").
A caveat, however...
If you are submitting to a short story market, it is usually best to simply submit to one magazine (e-zine, anthology) at a time. Yes, it does mean you will have to wait longer for a reply, but most of these markets won't waste their time with simultaneous subs (especially when a new author is involved). The website Writing World has an excellent discussion on this, if you're interested.
The best time to use simultaneous submissions is when submitting a novel to an agent. But, again, it is very important to let the agent know that you are doing this.
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