Do you setup your first book to allow for a sequal?

Always. I even make sure to do that with short stories. But I write series.

In the MS I just finished, I opened up a pretty big world. I introduced eight locations on five planets, a few specific subcultures, an alien race and a government facing a larger problem.

I didn't go into most of this, because I was telling one story about one family. I tried to tell my story as completely as I could (success level: I tried?) but there's a lot out there to explore.

To answer your question properly, I don't think I set up a sequel, but I do think I allowed for one. I don't want anyone feeling cheated that this story wasn't satisfying enough because I left big question marks. There are still things in the world I can (and hope to) dig into, because they are cool, and there because there are lots of other people who have stories to tell. So... no, but yes?

Nope. All my books are stand-alone. My attention span isn't long enough to write series.

But that doesn't mean everything is completely, perfectly wrapped up at the end. I leave one or two loose ends all the time.

No, but then again I tend to write standalones. Very rarely do I not, and if not, the idea for a subsequent book will usually come to me while I'm in the middle of writing the first one. Otherwise I write it with the intention of wrapping it up completely.

Sequel, yes. Sequal, not so much.

I generally can tell which story ideas would work as a series and which ones would be standalone. If there's a lot of world building going into the story, then it's likely going to be part of a series. So I do leave a few subplots or plot threads to be continued, because I prefer my series to be interlocked and the books to build off one another.

(I'm also a big fan of foreshadowing, so I like to drop little hints here and there that reappear in later books.)

ETA: Though the rule of thumb is when you're querying a debut the story--even if you want it to be book #1 in a series--has to stand on its own. (Tiny loose ends are okay.) Because there isn't a guarantee that if you do sell the book, a publisher is going to want the book to become the first in a series.

Not intentionally. Far too often I've so many ideas I want to write about, stories and characters to explore, things to see, people to do that there's no way to get everything into one book unless it's a zillion pages long.

It depends if I see the book I'm working on as a series, which most of mine are, so yes, I leave the book open for a sequel, but always make sure the book also reads as a stand alone (in other words the main plot has been resolved, but either a subplot is what will lead into the next book, or there is an OH SHIT moment at the very end that propels the story into the next. ). However, the contemporary and YA slasher/horror I'm currently working on are stand-alones that I don't see a sequel to. Like I said, depends on the story I'm writing and need to tell.

If you mean setting up a sequel as in leaving a cliff-hanger type ending, no, I definitely do not do that.
If you mean setting up a sequel as leaving a world that can be further explored, some foreshadowing, characters that could come back for a bigger part, etc. then I usually do something like that and leave an opening in case I want to come back and write more about that world/characters/etc.

If a standalone is rich enough, there is usually room for a sequel.

So what if you intend never to write a sequel? Your fans or publisher may get after you so much you find yourself returning to the world and people you created over and over till one day inspiration strikes.

Often you'll find a secondary character provides that inspiration, but it can be other elements of the original book too.

Generally no, but I don't tend to write series.

ETA: I do like books that have a hint of unfinished-ness in the end though. Some things are fun to speculate about after a book.

I write about people (fictional people), and their lives don't stop because this story is over. I may not ever write the sequel, but I almost always could for the above reason. This assumes I had an idea for one that I felt would make a good story. Sometimes I do, sometimes not. That doesn't mean I won't ever come up with one though.

Nah, I'm not much of a sequels person.

However, I do lean towards the more messy, ambiguous ending than gift-wrapped perfection. It's the lit major in me, I guess. I like satisfying, imperfect resolutions.

Usually I try to write it as a standalone with potential for a sequel (wow, sounding like a query letter here). No cliff-hanger endings, no major story arc left half unfinished, but a few loose ends and characters who could make a return in possible sequels.

It's actually a little hard for me to imagine a story that doesn't have a potential to be a series. Unless all the interesting characters die (even then, you could do a prequel), and all the minor characters are cookie-cutter boring, there should be enough there to make more stories if that's what you want to do. Every story should exist in a rich world that has more to it than the story that is currently being told.

Of course, there is a difference between having a rich world and setting up a series. And you certainly don't need to return to a world or character if you don't want to. But that potential always exists in my mind.

Well, I didn't. But the sequel started writing itself last month. And today, I discovered that one of the antagonists who died early on in the sequel has a daughter....

Totally depends on the individual book. Most of my sf/f work is written as "a stand-alone with series potential", and most of my contemp work is written as just a stand-alone. But then there are those lovely projects that demand to be different. For example, the book I'm writing now that is contemp yet meant to be the first in a series of stand-alone companion books... yeah. Totally depends on the individual book. How's that for a non-answer?

Yep, I did. I feel that even if you don't do it intentionally, there is always room for a spin off if not for a continuation.

Always. I'm not sure my brain even thinks in terms of books that are complete standalones. I can write plots that stand alone enough to be satisfying, but there's always more.

Of course, I write mostly fantasy. After I've built such an interesting world, I don't want to abandon it before I've written all the stories in it.

I seemed to have done that accidentally. Like it turns out two of my main characters are the same person. Then I ended up ended the second novelette on a cliff hanger. It become more like novelette duology with a cliff hanger.

Not sure I'll continue it, I'm kind of bored to a board writing SF.

Prequels, sequels, and spin-offs! Oh, my!

I think the trick is to write a 'complete' story—done in such a way as the reader is satisfied, but that they still keep thinking about it after you've rolled the credits. Allow the wiggle room.

I haven't thought about this in relation to my own writing (I feel like I've done good to write a whole manuscript without thinking beyond that), but I recall the story that "Ender's Game" was first published as a short story (or something less than novel length) in a magazine. Then it was rewritten as/expanded to a novel, and then it became a series. I kind of doubt it was planned like that from the start.

I'm not saying don't (or do) plan it that way, just that the "series" part could happen anyway.