In power exchange, a safeword is a word such as "red" which is not going to mistaken for pretend protest, and which indicates a limit has been reached. The scene either ends at that point, or is modified to avoid going over the limit.
You don’t need to have a special safeword if an ordinary “I’m ready to stop now” is sufficient. A special safeword is most useful in roleplay where someone might want to say “no! no! stop! stop!” without that actually meaning to stop.
You can also play without a safeword, in the sense of not being able to stop in the middle. This isn't all that uncommon in non-BDSM contexts: you can, for example, choose to go on a roller coaster ride, and then be stuck on the ride until the ride ends.
I sometimes come across a claim that playing without a safeword is edgier, more advanced than playing with a safeword. I've also heard submissives say that using a safeword feels like a failure. Neither of these are actually true. Let me explain.
A limit, as I describe in Limits and Surrender, is simple: Suppose you decide to do something, and X happens. If you had known that X would happen, if you had known your reaction to X, would you have chosen to do it? If the answer is “no”, then X is one of your limits.
So suppose we’re not using safewords, that is if there’s no way to stop in the middle.
What does that mean?
That means we need to figure out all the limits in advance.
And, if we do have a safeword, that gives us more flexibility. That means we can discover a limit during play. We don't need to have everything figured out in advance. We can get closer to the edge.
Not using a safeword intrinsically means that we need to be more conservative, if we care about not crossing limits.
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