Page 12 of the original DMG proposed a simple rule that became structurally intrinsic to my campaign, remembering that for me, a "campaign" is a full-on functioning world, providing a deep, resonant setting and structured logic enabling the players to innovate and self-build their own milieu. I appreciate that many DMs will argue that they just want to play a "game," that they don't want to spend that much time prepping their sessions or their campaign, and that they don't see the point in all this worldbuilding. Contrary to their point of view, for the players to be thoroughly invested, the game's interface MUST include a means for the PLAYER to prep for the session, and for the PLAYER to prep the campaign, in ways that push the DM to respond to what the player makes and what the player wants, rather than the "traditional reverse", which is all we ever see in most campaigns.
In my previous steady rewrite of the DMG, the content has largely paralleled my own campaign. There may be quibbling about when the dice are used, but my use of dice more or less remains close to the original game's intent. I maintain the old rolling of ability scores; I can see the benefit of using battle maps and representations on those maps. However, where it comes to "non-professional skills," the book and I totally depart company.
For one thing, all of the "skills" listed here are professions. All of them require considerable time to learn and excel at; no one can spend a few weeks learning how to become a miner or a tailor. These professions take years to learn well enough that an individual can employ the many skills involved with each competently. "Fisher" and "farmer" aren't "skills" ... they are occupations that incorporate hundreds of skills, none of which are listed in the DMG or even proposed. That failing in particular was acutely felt by me and my players whenever we spoke about this list: what on earth was the point in saying that the player was an armourer, a gambler or a trader, without also providing applications to these labels? Presumedly, Gygax did not think it worth the time; perhaps he felt that we could go to a library and look up what exactly a forester or a husbandman did. Of course we could do that — but since no book would explain how these things would work IN GAME, it was entirely worthless to include any reference, period.
As such, none of the campaigns I played in used the list. For a long time, not knowing what to do with it, I'd dutifully roll the result out with a wait and see approach — maybe, one day we'd figure it out. Later on, I added more professions. Steadily, I shaped in vague terms what a player could do ... and began to think on the problem, how do characters even get these skills, player and non-player alike?
What with things based on an historical period, we know that farmers gave birth to farmers and merchants gave birth to merchants. Lateral movement between professions occurred when a boy was adopted by others, not because his farming father decided to become a merchant. People were legally tied to the land, or to guilds, or deliberately kept out of organisations, much like you or I cannot join many associations or clubs because we don't have enough money in the bank, or we're just not the right kind of people. The rules then were far more draconian; even if you had the money to buy products and carry them elsewhere, you would still be denied entry to a town because you didn't have permission to sell here, or because you did not belong to the baker's guild in this town, though you might belong to a baker's guild elsewhere. These hoops were designed to force you to follow in the footsteps of your father, because the upper echelons didn't want competition. That didn't change until so much land and opportunity became available, through the discovery of new worlds and new trade routes, that the wealthy couldn't maintain their hegemony.
Calling these skills non-professional implies that the character classes, "cleric," "fighter," "thief" and so on are professions, which they're not. Religious persons do not self-identify as "religious persons," but as priests, friars, teachers, deacons and a host of other professions based on who manages, meets & greets, handles the money, performs what services within the religion's heirarchy and so on. No one has ever put on their resume, "fighter." Fighting is a set of skills, not a profession. They think of themselves as soldiers, guards, lancers, marines and a long list of names associated with specific jobs. Outsiders may think of a group of persons as thieves, assassins or monks, but that is not what they call themselves, as evidenced by names like "cutpurse," "burglar" or "forger," which are all about stealing but each having totally different skill-sets. Monks do not call themselves "monks" ... they call themselves Buddhists, Jains, Cistercians, Franciscans ... in short, by the set of rules they personally accept as their path to enlightenment. Paladins aren't "paladins." They're knights, crusaders, peers ... AND soldiers, noblemen and such. The word doesn't appear in English until 1592, and comes from the French word that means "of the palace." It only means what we think it means when we speak of D&D; it is a convenient word picked by Gygax and crew that has become a convenience, but is not accurate to anything historical.
The heading, then, deliberately calls things that ARE professions "non-professional," and things that AREN'T professions "professional." Gawd, is it any wonder we were soon immersed in people pulling words out of a thesaurus to make new D&D classes, since obviously a magic-user isn't a witch, a necromancer or a sorceror. Shoot me, please.
We didn't need more classes, we needed a rational skills list that could be employed by players in the campaign ... as evidenced by 2nd Edition's decision to do exactly that, because the demand was there and needed addressing. Unfortunately, while the skills list was fairly comprehensive, the scholarship behind them was utterly lacking, so that every skill was seen as a kind of saving throw determining success. I do not need to roll a die to write this post. Nor do I need to roll a die to make this sentence grammatically correct. Astronomers very rarely fail to have perfect knowledge of pre-telescope astronomy; so what is the "check" for? Gaming, obviously. The designers, having no concept of how D&D worked or even how skills worked, turned them into a silly game that subverted any value they might have had ... and the pattern was picked up by 3rd Edition with glorious incomprehension.
It's self-evident why this didn't work, to me at least. A game's random generating functions cannot overwhelm the predictability of the game. Randomly rolling snake eyes or box cars playing Monopoly, though very different numbers, is mitigated by the relative slight difference in the effective results. You're standing on Connecticut Avenue, and the full scope of results will land you anywhere between St. Charles Place and Kentucky. The difference in price between those properties is only $80 (St. Charles, $140 and Kentucky, $220). Unfortunately, D&D checks do not offer slivers of success, but total success vs. either total failure or partial failure. The "greyer" the partial flavour, and the more kinds of partial flavour, the more practical the die roll becomes for a game.
Combat presumes that in most cases, you won't die in one round; low-level combats in my game typically last for no more than 8 or 9 rounds, and then only if the baddies don't go down more quickly. Typically, it takes at least 8 to 9 rounds to kill a party; this gives them time to explore strategies and decide how much of their full strength they want to reserve for a later fight. As turn 7 passes, with the players still doing poorly, the party stops counting on swings and starts going all-in with their magic and one-shot options.
2nd Edition and 3rd Edition systems didn't account for any greyness; and constant, inconsistent pass/fail results forced players not to rely on skills, except when failure didn't matter; and if it didn't matter, it didn't add to the game, so why have the system? Only the fluff-lovers could embrace a system that worked inconsistently when it was really needed and didn't provide meaningful, trustworthy game play ... and it is always the fluff-lovers still who defend those systems (or similar systems found in later editions).
Full disclosure, I have similar checks in my game. I also treat them as "saving throws" ... but as Pandred pointed out a couple of weeks ago, rolls against these things are rare, and CAN be avoided if the players take steps to protect themselves, or don't take risks. This is different from a skill roll, such as Lore, that isn't actually needed for the game being presented. Lore is frosting; and if we don't have it, meh. We still have the cake, that being the actual campaign, and what we don't know about the lore and need to know, the DM will eventually provide.
A skill set that provides game flavour has to also be reliable ... as reliable as knowing that when you buy food to keep you from starving, there's no die roll to determine if it does. Generally, games don't roll dice to see if the horse will carry you to the next town, or if the rope can be let down from a wall to help you up. If we did roll for these things, we'd quickly see players stop using horses and ropes. Players organize their strategies on things they don't have to roll dice to accomplish ... and while there is a sick conception that giving players skill sets that don't need rolls would make them "too powerful," truth is that the more consistent, reliable functions the game offers, the more opportunities and dimensions the game offers.
This post is long enough for the present; let's come around back to where the skills come from with the next post.

