Co-op: our favourite abbreviation. We're not simply talking about your basic multiplayer here, and nor are we singing odes to teamplay in Counter-Strike or World of Warcraft. We're talking co-operation. Two gamers versus a game. That is where the very best gaming moments lie.
But why, given the unusual pleasures of co-op, and its recent surge in popularity, have developers only recently faced up to developing for two players rather than one? Could it be - whisper it - that's it's just too damn difficult?
The joy of co-op is like the joy of conversation. You're not competing - you're working on problems together. Also like conversation, a good co-op session spontaneously generates ideas that you wouldn't have had on your own.
In playing against the computer with a friend, you come up with different and often wholly more entertaining solutions to the problems that games present you with. Whether it's fighting Nazis in Brothers in Arms, or purging a nightmarish hive in Alien Swarm, there's simply nothing better than coming up with a plan, and then saving your best mate when it all goes horribly wrong.
Despite our love for it, co-op has had a patchy history, and there's a crucial reason for this: it's really hard for developers to pull off successfully. The problems for the people making these games have only become worse over the last decade. The issue is one of complexity.
The simpler games, such as Quake or Doom, didn't really create many problems for designers in terms of implementing co-op. They were essentially just rooms with monsters and traps, so all they had to do was drop in a second player model and Bob's your uncle sitting next to you with his PC linked up to yours.
But as games began to learn the lessons of Half-Life and Tomb Raider, such simple solutions became impossible. When an experience was ever-so-carefully scripted for the enjoyment of a single player, dropping in co-op - and a second player - became a task too far. Games broke down, enjoyment faltered, and the whole exercise became too much effort to bother with.
Today, it seems that developers are beginning to crack the problem. Not only that, but their ambitions are bolder and their intention to provide co-op far clearer. Where games have long bolted co-op on as an afterthought or a neat alternative multiplayer option, new games such as Kane & Lynch, Left 4 Dead and Gears of War have taken co-op as their central motif.
With a second character on hand, often controlled by smart AI during a singleplayer session, there's an option for player two to drop in and play through a level with their chum. It's not just a case of adding a model for player two, it's a matter of making the second character absolutely integral to how the game works. How long, we wonder, before the big shots like Half-Life 2 have a second playable protagonist?
Developers, many of whom used to shudder when we asked them about co-op, are now warming to the idea. Harvey 'Deus Ex' Smith became rather animated when talking about BlackSite's proposed drop-in/drop-out co-op mode, although he was happy to admit that it was a genuinely tough problem: "We started off being so arrogant that we said "oh, co-op will be easy" but then at every turn it has fucked us.
"It's amazing how you can be in a room full of MIT grads, and people who have been working on games for ten years and play all the games, and look at the helicopter with one turret on it and then someone finally says 'What about player two?'"