Quick: what's the biggest difference between console and PC games? No doubt a lot of things spring to mind, but here's one you might not have thought of right off: any good PC game soon attracts fans willing to spend time learning, and then altering the games they love. Are fan modifications, or "mods" the ultimate act of fan worship? Or a pain in the neck for developers? Depends who you ask.

Mods have been with us for years, but seem to be getting more attention than ever, both positive and negative. The Counter-Strike mod for the game Half-Life has been the multiplayer game of choice in online shooter circles for several months now. Billed by one reviewer as "the perfect balance between realistic tactical combat and no-holds-barred fun," Counter-Strike and similar Half-Life mods are helping extend the shelf-life of a game that otherwise might have fallen off gamers' "must buy" list months ago. On the other hand, the shutting down last week of at least two fan projects by LucasArts shows that mods are not only not universally accepted, they're sometimes even seen as threatening.

Many of today's leading developers of first-person shooters even started out as amateur designers. For example, now-established franchises like TeamFortress (designed by Australians Robin Walker and John Cook) began as mods to commercial games.

Norwegian Sverre Kvernmo, the lead designer for KISS: Psycho Circus, also began his career doing up new map levels for Doom (one of the first mod-friendly game designs), before "going pro." "For me, a major lure of mods was to see your own creations come to life and have other people get a kick out of playing them," he says. "One of the coolest feelings is when you somehow supercede the originators' performance. It's not a fair comparison of course: the developers can't possibly explore every aspect of a game they make, but it is gratifying and makes you feel like you're modifying a game that has some depth."

The lure of doing mods to a beginning game designer is obvious, he says. "Mapping for a hobby you generally have a fully working game to mod, or 'a full pallette.' When you try and make games from scratch there is less time spent in the 'fun zone', creating actual gameplay, and more time fussing, tweaking and engineering."

There are, generally, four reasons that fans start modifying or altering their PC games. Some changes are made to make the game more personalized, or to increase player identification, such as adding new character portraits in Baldur's Gate, or flight simulator "skins" with the markings on one's favourite squadron or ace. Other modifiers seek to expand the scope or breadth of a game, to realms that were not incorporated in the original product: the adding of entirely new aircraft or scenery to flight sims, or first-person shooter mods such as Half-Life: Counter-strike would fall in this category. In most cases, designers see these as innocuous, even useful in extending their game's shelf-life: one even sometimes sees boxed mod sets for games like Microsoft Flight Simulator sold on shelves next to the original product.

The other reasons can be dicier. Some people modify games to fix "mistakes" they've found, or feel they've found, in the original product. This can be to enhance their realism, or their playability value, or both: giving more protection to units who seem to die too easily, or making a plane fly as fast at a given altitude as the books say. Mods like these are more common the more a game claims to be reality-based: the bleeding-edge flight sim market, or the "historically accurate" wargame market, and are generally connected with a thriving online community of fan/critics. But the worst kind, and the one disliked by fans and developers alike, are alterations to data files, etc., designed to give one player a hidden edge in online play: increasing the armour or ammo load on a tank, for instance. While less common now, there's no doubt it has been a serious concern for certain online gaming communities. Making a game easy to modification risks also leaving it open to those kinds of abuses.

Another risk in making a game's architecture more open to alterations is that it can become more transparent to commercial competitors. Game developers who brave the discussion boards dedicated to their games often have to toe a fine line between disclosing their thinking to fellow game-lovers, and keeping possibly proprietal information within the company. Whatever the fans know, or can figure out, can also be got at by someone wanting to make their own, possibly competing product.

As a rule, FPS developers have the fewest problems with amateur modifiers that work within the systems they've been given, sometimes even poaching the most talented modders as level editors: id Software, makers of Doom and Quake, are famously open to amateur modifications. But other genres and companies are not always as forgiving.

Perhaps the company with the most "mod-unfriendly" reputation is LucasArts, which sees fan add-ons to its line of adventure games as a threat to the sanctity of its intellectual property. Early this month, LucasArts lawyers moved to shut down two fan projects related to its Monkey Island game line. In its cease-and-desist letter to British amateur designer Matt Shaw, LucasArts Legal said all such use of copyrighted material or characters was strictly verboten.

"It has come to our attention that you are creating a Monkey Island prequel video game which involves characters, story elements, and/or locations from LucasArts' Monkey Island computer game series... Regrettably, we must inform you that you can not include any such items in any of your projects. Lucasfilm Ltd. And LucasArts Entertainment Company LLC are the sole and exclusive owners in and to all elements of our software titles, and permission to use them is seldom granted."

Websites for both Shaw's Legends of LeChuck and the Fate of Monkey Island 2 were promptly closed down, and all mod-related content removed. In a letter to supporters, Shaw said he's "decided it would be best to have a break from the LucasArts community" for an indefinite period.

While LucasArts has been the most aggressive thus far, other developers are also seen as mod-averse. The flight-simulator genre is particularly mixed, says Minnesota programmer and flight sim enthusiast Brandon Bohn, with some firms tacitly encouraging their fans to experiment and create, and others offering little or no support. But in this genre at least, games that do offer that kind of encouragement tend to enjoy more lasting support, Bohn believes.

"Microsoft Flight Simulator is one example. The game in itself would probably have died a long time ago, but all the new stuff coming out from third parties are increasing the [shelf] life... Falcon 4.0 is still getting sales from new people buying the game day after day, because they see the product still has a life."

Other developers who are less friendly to modifications pay a price, says Bohn.

"Jane's/Electronic Arts is one such company. They lock up their code, and nobody can modify anything in their games, and so their products have a short lifecycle. If its data files had been open to people, people might still be playing U.S. Navy Fighters, and if those changes required buying the original, Jane's would still be getting sales."

On the discussion boards at Combatsim.com, there does appear to be a definite difference. Microprose's European Air War was released at the same time as Jane's graphically superior World War 2 Fighters. Total postings by fans on the WW2F discussion board are in the 1,300s right now. Posts for EAW (many of which concern new modifications and mod-related questions) just passed 32,000.

Jim Dattilo, who writes about flight sims for About.com, says it's a principle that should be applied cross-industry. "Hard-core sim companies are starting to realize that opening the code and allowing controlled mods will only contribute to the longevity of their sim. [Even] non-flight sims like The Sims or SimCity thrive on user-created skins and buildings... the huge popularity of these games would be greatly diminished if user creations were not possible."

While encouraging cheating has sometimes been the result for flight sim fans, in at least one case it proved self-correcting, says Dattilo. "Some Microsoft Combat Flight Simulator players were using programs to auto-follow enemies and therefore make it simple to shoot them down... a third party created a program that blocked the use of the auto-follow cheat. Microsoft never intervened one way or the other."

Occasionally, however, game developers still feel compelled to draw the line: a notable case in point is Atomic Games, makers of the Close Combat series.

The relationship between Atomic and its die-hard fans through the first four Close Combats (which focus on real-time squad-level WW2 combat) have been famously antagonistic. By staking out the "ultra-realistic" niche in their genre, Atomic set themselves up as the favoured game, and the favoured target, for the predictable crew of historical perfectionists. Normally, these fans would just fill up discussion boards with the historical discrepancies they found, but with Close Combat there was another outlet... the data (armour, speed, ammunition) was all contained in easily editable data files and vehicle textures, that welcomed extensive player modification. Was that the wrong kind of German tank at Arnhem? Change it! A strong editing community soon sprung up online, adding vehicles, "fixing" stats, and generally messing with the files.

That, however, would lead to problems with Close Combat 3: The Russian Front, which while enjoyed by many, was criticized by some historical realists and playability fans as too weighted in favour of armoured units. One team of fans, led by Ron Gretz, created "Real Red," a massive modification to fix the problems they saw.

Gretz is a firm believer that easily modifiable games are a definite bonus for the diehard wargame fan. "I still see people playing Panzer General 10 years later due to the mods available even though it is a somewhat archaic turn-based system; due primarily to the play-balanced data mod and the many scenarios that have come out in the 90s for it."

Their intentions may have been good, but Atomic president Keith Zabalaoui says he found it difficult not to take the implied questions 'Real Red' raised about Atomic's work as an attack on the company.

"Groups such as 'Real Red' hacked our data and arbitrarily decided that it was buggy. The great fallacy was that they didn't know what they were looking at. Despite our online explanations and our private conversations with 'Real Red' members and others, the impression that our data was flawed was still out there."

When it was time for the next installment in the series, Close Combat: Battle of the Bulge (also known as CC4), Atomic made some sweeping changes, switching parent companies from Microsoft to SSI and adding data compression to the previously easy-to-access data files. That would lead to new accusations from the fans, that Atomic was "encrypting" the files to keep them quiet, accusations Atomic staff didn't work too hard to deny, frankly.

"Our original reasoning was that the text files were slower to load into memory," recalls Zabalaoui. "[But] when we finally did convert the data files to a non-text format, people complained that we were shutting them out from hacking our data. That was not the reason we did it, but it was a side effect for a majority of folks out there."

"After the experiences I described [with Close Combat 3], I was not at all unhappy about that side effect."

The change was welcomed by some fans of online play, such as Ross "Future" Moorhouse. "With CC3 and all its mods, I have heard of complaints from people who have trouble playing others because of all the versions of mods. Some people don't update their games with the latest mods. With CC4 this isn't a problem anymore."

Gretz concedes that compression did have its advantages for online play, not just in eradicating cheating. "You have to remember in the heyday of CC2 and CC3 multiplay, there were numerous complaints about speed. People forget that machines were still running on 486s and low-end Pentiums (66s and 100s) on low-speed modems. Data compression then would have made a world of difference."

And for the real diehard modifiers, compression was a minor problem, he adds. 'When I first heard about the change in data structure in CC4, I know someone would figure out the correct compression algorithm and that tools would arrive soon after that." One thing the Close Combat modifiers aren't doing is going away, he adds. "The best part of modding, is that it is growing the core gamer constituency. As the older modders "burn out", they are being replaced with new 'like minded' players who want to continue expanding the games. This is good for everybody concerned."

Robert "Cappy-R" Ellison, another well-known Close Combat modifier, says Atomic should take the dedication of its fans as an indication of the game's quality. "I have seen many other games that would have been easily modifiable, but their engines were so lacklustre, what would the point be? Close Combat has a great base engine, and that is why people love modding it." For Ellison, the added obstacles just made his job more interesting. we just have to fully crack the game now instead of having it editable out of the box. If you ask me it brings a bit more excitement."

But does that mean game designers should work to make their games more "mod-friendly?" Or are the resources spent helping fans who've already bought their game better spent in putting out the next one instead? Zabalaoui, for one, says the numbers of fans who use mods is often almost inconsequential to a game developer.

"I also believe that only a teeny-tiny fraction of fans ever make their own mods," says Zabalaoui, although he concedes a somewhat larger number actually play them. "The exact numbers for these groups are unknowable... we can only speak in fuzzy terms. [But] all told, the Close Combat series has sold in the neighborhood of 2 million units. So, if we say, 1,000 of those people are mod-makers (an extremely high number, in my opinion) and 5,000 are mod-players, then we have 6,000 people who would be interested in mods versus 1,994,000 who are not.

"The upshot," says Atomic's president, "is that I can't justify going out of my way to incorporate editing capabilities into my games. That's not to say that I won't do it. If we can accommodate those 6,000 people, without it being a "tail wagging the dog" situation, then we'll do it. Bottom line: user mods should be supported but should not drive the game's design. They should be subsidiary to just about every other concern... including design concerns, scheduling, and cash flow.

"I want to make games, not tools for others to make games."

But Ellison says Zabalaoui's missing the point. "The figures don't say as much as the work put into the mods... it doesn't matter how many people actually play them... it is how many might like to play them if they knew."

The Close Combat series itself has gotten some new competition this month, with the release of Big Time Software's Combat Mission. Both games share similar topic matter and the same spiritual heir (the old Avalon Hill boardgame Squad Leader) - one big difference, however, is their apparent interest in fan modification. Big Time president Charles Moylan has said in media interviews that making Combat Mission more open and extensible for fan alterations is a key feature in the new game's design.

To mod or not to mod? Will the rules of the marketplace win out, with the gaming public voting with their feet for games they can change to suit themselves? Probably not: quality of graphics, gameplay, story, fun, all likely have a larger influence on sales. The mod community for any game remains a small portion of its overall buyership, and, issues of online cheating and intellectual property rights aside, the sheer and growing complexity of games means that those who dare to look under the hood will continue to be a small portion. After all, the console game industry thrives quite adequately without any user mods at all.

On the other hand, those small groups of diehards aren't going away, either. Many of today's leading gaming professionals cut their teeth on modifications they made themselves, learning computer and storytelling skills in the process (not to mention marketing, customer service, fan support) that are not, maybe cannot be, taught in schools. No, the curious relationship of gaming companies to these friendly pirates will likely continue. Existing companies will likely always have the same choice: to regard that latest talented modmaker as a future prospect, or a future threat?