GURPS has always had rules for attacking through occupied hexes.
The rule goes all the way back to Man-to-Man, p. 30 (sidebar).
In Man-to-Man, you had all of two Reach 2 one-handed weapons - the bastard sword (counting both variations as one) and the rapier. The other 2- and 3-hex weapons? Polearms of all sorts, two-handed axe/mace weapons, staves, and spears.
These days with GURPS, you have a lot more options for Reach 2+ melee weapons, from edged rapiers to whips and kusari and two-handed weapons wielded in one hand by overstrength users.
Let me just say I never really loved this rule.
It makes a lot of sense for certain circumstances:
- formation fighting.
- pole arms held in two hands.
- thrusting attacks with the same.
It makes a little less sense for others:
- random melees where fighters just happen get between a friend with a long weapon and a foe.
- swinging attacks.
And almost no sense for others:
- long flexible weapons.
- tight quarters.
I just don't find it plausible that you can freely attack through your friend's hexes, without penalty to you or penalty to them, with any long weapon. The line in the books is "You may attack through friendly characters at no penalty (this is a basic part of your training with any long weapon)." With a polearm, I can buy that. With a staff, maybe. But a rapier? Did they teach stabbing past friends?
What about the person being attacked past? It seems like if I said, "I'll stab people from behind you while you fight. You've never had someone do that, but don't worry, it's part of my basic training with my weapon." It seems like it would interfere with you at least a little, or I'd have to limit my actions to things that I was sure wouldn't have any chance of interfering. If I did that, why am I at no penalty?
We've ended up with results I find really hard to believe, myself. This are these old favorites:
- Formation Fencing. Front rank, heavy fighter with a large shield, high DR, and a Reach 1 weapon. Behind him, a light fencer with a Rapier with Reach 1,2. Stand behind and stab away!
- Unlimited target selection. You can strike with a halberd held at Reach 2 and swing and hit the vitals of a person in front of your friend. Heck, you can strike his foot if you wanted! Hard to suspend disbelief moments include successfully stabbing past your SM+1 friend with his large shield into a human-sized target's eye in a one-hex-wide corridor.
- Perfect coordination. I've only seen this a few times, but it's a highlight reel replay. Front guy goes. Back guy goes, then steps back, leaving a clear hex. Front guy gets attacked and, if necessary, Retreats into the vacated space he senses is open. If he doesn't need to Retreat, the back-ranked fencer dude does Committed Attack to get two steps, and steps, stabs, and steps back. Full defensive flexibility is assured, without any wasted time coordinating with each other!
- Feint from cover! This one is especially technically possible if you treat "Feint" as an optional use of an Attack. You can stand behind your friend and Feint against people in front of him. After all, you can feint against anything you can reach to attack, and if the attack is at no penalty than the feint is, too. Now just imagine it as a Beat. It's plausible that an attack from a second-rank fighter is trickier to stop because you can't read as much body motion (he's obscured by his friend), but equally the second-rank fighter has many less options for trickery because your opponent must be aware of what you're doing to fall for it.
- Far Side, Close Combat. This one is standing behind your friend who is in close combat, and attacking into close combat to hit the person in front of him. Makes sense when the target is larger, sometimes when they are the same size, but it's just weird when you Rapid Strike with feints and slashes at a guy who is choking your friend that you're standing behind.
I've generally ruled in a mishmash fashion. Basically, no penalty to hit, but lots of situation rulings to keep people from stabbing the vitals, chopping up foes with torso slashes with one-handed long swords, and so on.
But what if I just changed the basic rule? Here are several ways to change it.
Long Weapons and Occupied Hexes
You can attack through a friendly-occupied hex with no penalty if you have a long weapon held in two hands that has Reach 2 or more. Not all hit locations will be valid targets!
Slightly more complex versions:
One-Handed Penalties: As above, but one-handed weapon with Reach 2+ can also attack through friendly-occupied hexes, but suffers a -4.
Thrusting Only! As above, but you can only use a thrusting attack. (Great for spears, pushing with staves, etc., and makes reverse-gripped greatswords work really well from the back rank of a melee!)
Thrusting preferred! As above, but thrusting attacks are unpenalized. Swinging attacks are more awkward, and do -2 damage or -1 per die, whichever is worse (note that is functionally identical to the penalty for long swings with All-Out Attack (Long))
Harsh Realism: It's not easy to attack past your friends, if you haven't trained with each other to do so. Attacks through friendly occupied hexes are at -4 if you want to avoid inconveniencing your own friend, or you can attack without the penalty but if you miss or your target Dodges, you are considered to be Striking Into Close Combat (p. B392) against your friend! The Teamwork perk - if you both have the same one and are formed up - negates these penalties. (Best combined with another of the options above - thrusting preferred or thrusting only.)
How I'd run it? Harsh Realism seems like the most fun. "Don't worry, I'm sure I won't miss and then roll a 9 or less, I swing my sword for 4d+12! Oh, he Dodged." The simple one is "two handed weapons, thrusting preferred.
It's probably easy to see this as "nerf the swashbucklers," but it's not really intended to be that. It's more like "close the weird reality-warping loophole that swashbucklers have been slashing through" more than anything else. And it might just be easier to have set yes/no rules (two-handed only, thrusting only) or easy to remember options (two-handed only, harsh realism) than to make ad hoc rulings each time about the validity of cutting off the tentacle that's grappled your friend from the far side or stabbing past your friend's arm and shield and into a foe's eye without bothering your friend.
I'll have to see what my players think, of course, but they might like a clear set of "yes" and "no" over a lot of case-by-case maybes.