One thing I like a lot about GURPS, ever since Man-to-Man, is that weapon selection matters. What you use to fight can heavily influence your options and opportunities. Weapons make styles, and styles make fights.
Some of those weapons come with their own specific utility. So I'm going to do a series of posts on some of what I consider especially interesting weapons in GURPS. That is, weapons that have some particular use or utility that warrants special consideration, special tactics, or special rules. They often give up something - often a lot of something - to fit some specific role better. This isn't a rules classification, it's more of a way of thinking about the different utility of some weapons. Today I'll look at one I like a lot.
A classic. Flails seem pretty straight-up weapons in GURPS, but they have some special rules attached.
Flails all come with a set of defense penalties for your opponent. -4 to Parry, -2 to Block (halved for the smaller ones, like nunchaku or weighted scarves), cannot be parried by knives or by fencing weapons. Your only fool-proof defense against them is to Dodge.
They also come with pretty good damage - a benefit to having a chain section connecting a lever and an impact surface. For two-handed flails, your selection is pretty small - either a flail or a three-part staff (aka three-section staff). For one-handed flails, you've got a bigger set of choices. The king of those is the morninstar, which gives swing+3/crushing, which is pretty much the top end of one-handed damage ranged. Only the mace matches it one-handed and it doesn't give you nearly as much. They're all cheap, too - $80 for a morningstar (which is 4x as much as the next closest one-handed flail), $100 for a two-handed flail (and only $60 for a three-section staff if you want to go all Gordon Liu in combat.)
What makes this a special weapon choice? For starters, it's DX/Hard, you don't get a wide variety of weapon choices. It doesn't give a lot of useful defaults (Axe/Mace, Kusari for two-handed flails, and the other flail skill, that's it). You'll need ST 12, too, for a morningstar, or 13 for a flail, which isn't that high but it's not low, either. ST 10 or less and you're down to the half-penalty nunchaku and the various less effective varieties. Flails are mostly Parry 0U (a few are -2U) so you'll want a shield or an off-hand weapon to protect yourself with.
That's another point right there - your advantage with a flail is purely offense. Unless you're playing DF and have the Dwarven modifier on it, you can't attack and parry on the same turn. Your advantages with the weapon are damage and imposing defense penalties, so don't use it to defend. Stay close, think about Committed Attack for the extra step if you get hassled by Reach 1,2 weapon-using opponents (especially fencers), and think about armoring up. This isn't a light infantry weapon.
Don't forget about Defensive Attack*, either. You can trade off a little of that extra damage for a chance to Parry on your own turn. You can still act aggressively, just hold a bit back to make sure you aren't defenseless. This is especially useful for Weapon Master types (who get a discount on multiple parries and bonus damage to offset the tradeoff.)
Perhaps surprisingly, given that it's DX/Hard, both the One-Handed Flail and Two-Handed Flail skills are ideal for lower-skill fighters. The aforementioned penalties to defend against them make them ideal for guys who can't afford to trade off a lot of skill for Deceptive Attack. But you only trade off 1 point of relative skill compared to a DX/Average skill (in other words, most weapon skills) for a high-damage weapons that imparts steep defense penalties. Score! You can even use Telegraphic Attack and only partly offset the parry penalty and/or to just paste low-Dodge fencers, who don't get to Parry no matter how many pluses you give them to their weapon.
Flails do swing/crushing damage**, though, so they don't get any multipliers for location except for pretty difficult and easily armored targets - Skull is good for x4, but easily armored up, Neck is x1.5, but hard to hit and easily armored up, Vitals just doesn't give you all that much, for examples. You'll probably want to hit for these kind of places anyway, if you have the skill, or just take a random roll. They make a great choice for people so strong they don't need or want to hit specific locations beyond "center mass" anyway, like giants.
Your goal with the flail should be to minimize the chances your opponent can Dodge and Retreat, since that's their best defense against you. As with all weapons in GURPS (and reality), being stronger and more skilled will make it work better for you. But flails give you a bit of a jump on both, by adding in some defense penalties to your opponents and giving you solid damage for only a small skill tradeoff. Point for point you'll lag behind a swordsman or an axeman or spearman, but you'll always be just a little harder to stop.
Full disclosure: Both I (with Tarjan Telnar, my DF knight) and the PC in my game Honus are flail-users. Me because I think morningstars are awesome and made for a fun knight, Honus because it's a high-damage weapon that imparts defense penalties so he's not left behind by more skilled fellow PCs. Please note that I didn't address the kusari and its derivatives - they have a lot of their own special rules and cases that don't overlap with flails. I'll talk about them another time.
* Thanks Joseph R for pointing this out in the comments.
** Well, unless you allow the very silly Bladed Flail from Low-Tech Companion 2. If you do, you're asking for it. It's in the cinematic weapons section with double-bladed swords and barbed chains for a reason.