I have a character who always carries a knife concealed on her person. But where should she keep it? (In other words, where are the best places to wear a hidden knife and what factors might you consider in choosing between them?)
There are a couple considerations for concealed weapons. I know you’re asking about knives, but a lot of this also applies to concealing weapons in general.
Who your character is and where they operate has a huge influence on what they need to consider when deciding where, and how, to conceal a weapon.
Similarly, their clothes are a major factor in their options for concealment. A character with a lose fitting jacket, has a world of options that don’t exist for someone wearing, a restrictive formal dress.
For example, a mercenary or law enforcer wouldn’t need to worry about being seen with a small weapon in reserve, so they could settle for something that’s relatively visible, but might be missed at a glance.
If your character has a weapon hidden across the small of their back, that’s going to be very accessible, but it may be visible if someone knows to look. In this specific example, your character has an advantage in that she’s a woman. The curve of her spine at the back makes it easier to conceal a weapon there without it leaving a visible bulge while wearing a waist length jacket.
One of my favorite bad ideas for concealed weapons is the hidden blade from Assassin’s Creed. This is a retractable wrist blade which mount along the user’s inner forearm and extends into a punch dagger. It’s not a bad idea because it’s a poor place to hide a knife. In fact, it’s an excellent place to hide one. Hiding a weapon up your sleeve was an effective tactic and it led to practices such as the hand shake, which used to be a weapons check. You’d shake someone’s hand while gripping their forearm to make sure they didn’t have a knife hidden up there.
Hiding guns up your sleeve is a little less viable. There have been a number of experiments on the subject, but the results lacked both reliability and stopping power. If you’re going to hide a gun up your sleeve it’s going to be small, and your best option is to use it to kill an attacker and take their weapon. (Which was the intended use for the glove gun, incidentally.)
So, up to this point, I’ve been prioritizing access. Accessibility is an important consideration, because if you need the weapon quickly, it needs to be someplace you can reach easily. However, if their primary concern is hiding the weapon, then it’s possible they might have it concealed someplace far less obvious. She could hide a weapon against her leg, under a skirt, and it would be fairly difficult to spot, but getting to it could be quite awkward. Similarly, boot knives and ankle holsters are an option, but they do require you to have easy access to your shin. Those last two are easier to reach from a sitting position, rather than standing, so this can inform when you’d want to hide something there.
Another option, which runs contrary to what you’re looking for, is concealing the weapon in the environment ahead of time. This an option if your character will be searched for weapons on the way in. Any half-security team will do a cursory search for weapons ahead of time (if they can), but it’s possible there may be weaknesses that would allow someone to get a weapon into position for your character. (Obviously, this doesn’t work if they’re operating alone.)
Finally, there’s a very consideration with knives; folding knives are a real thing. It’s possible that your character would have a collapsible knife, or something similar. In a science fiction or fantasy setting, it’s entirely possible your character would have a device (or enchanted item) which creates the knife itself. So your character wouldn’t be hiding the weapon, she’d be wearing a piece of jewelry that creates the weapon in the moment. Of course, if we’re talking about fantasy, it’s entirely possible your character can outright conjure a blade from the aether, so that might be a possibility as well.
On a less fantastic note, there’s also tactical batons, and much like folding knives, these collapse into little more than a handle. These are pretty easy to hide in a pouch or pocket, so you have a lot of options for where she could conceal these.
I’m realizing now, some of the difficulty I have with this question is, the answer changes. I mean, I carry a knife (most of the time), but I don’t carry it in a single place. Sometimes it’s a pants/jeans pocket, sometimes it’s in my jacket. Sometimes it’s in a satchel on my person. There isn’t one answer here, and it can change whenever I’ve been using and put it away. Also, the knife itself changes, I’m a walking continuity error in that sense. I’ve got three lock-blade box cutters on my desk. I don’t carry them as self-defense weapons, I carry them as tools, and whichever one is most convenient is going to get picked up if I need it.
This is something that’s generally discouraged with characters, because it’s more chaotic, but it is true to life. If your character always has a knife, it’s entirely reasonable for her to have it hidden in different places at various times. It’s not like there’s only one possible place she could hide it. It’s also entirely possible that she has more than one.
You have a lot of options here, the major considerations are, “how easily can she get to it?” and, “how well hidden is it?” Once you have those two considerations in mind, the rest is going to be looking at the context.