At a lot of game tables, people use different weight values for a gold piece. It's one of those things people tinker with almost as much as the thief guy class. After all, historically, gold pieces weighed between 1/30 and 1/50 of a lb. Gold is also much more common in D&D than it was in the real world. People guess that prices in D&D are high by a factor of ten over their real world Middle Aged counterparts. Anyway coins weighing 1/10 lb. aren't particularly realistic. For instance, you'd need 55 or so $1 coins to weigh one pound.
Brother, let me tell you: 1 GP = 1/10 lb. is gospel to me.
In search of "realism," the game-mechanical value of 1 GP = 1/10 lb. is lost. (I say "realism" because this is a game of magic and dragons and other dimensions and lady elfs who walk around in plate armor and high heels for 16 hours a day.)
Then instead of weighing all the other stuff you use in lbs. or whatever normal people use as a unit of measurement, Gary* goes and compares the weight of everything from a clove of garlic to a suit of plate mail to an equivalent weight in coins. What?
So why did Gary tell us we were hauling around coins the size of dinner plates? And why weigh everything else in coins? Here's why, friend:
The object of the game is to drag as much money out of the dungeon as possible.
Coins are the measure of wealth.
Wealth is XP.
Not only that, but gear is XP too. Plate mail is XP. Pole arms are XP. That third or fourth treasure sack you got there? Totally XP.
Every piece of equipment you carry is that many fewer coins you can potentially haul out. Is that extra 50' of rope worth the money you have to leave in order to carry it? And tangentially, is it worth it to hire a lazy cowardly old stevedore for 100 GP? Will it pay off in more than 100 GP in extra treasure? You have to "weigh" the cost now against the possible benefit later.
But 1 GP = 1/10 lb. gets even better.
Then, Gary measures progress in distance against weight carried - encumbrance. Again, there is a direct proportional relationship between game progress (GP = XP) and physical progress through the dungeon and wilderlands.
Gary really made it very simple for us, didn't he?
- He told us exactly what is important.
- He set the fundamental unit of game progress (XP).
- He translated everything else in the game into that unit of measurement (1 XP = 1 GP) so
- we can clearly see the costs and benefits of each logistical change we make.
10 GP weighing a pound doesn't seem much like the "real world" but it's a lot easier to imagine a world with common gold than it is to fiddle with all the numbers and make it work out as well as he did it.
*And Dave and Rob and Michael and the several others who were there first and did it right the first time.
Full disclosure: In Treasure Hunters Prolix, I made coins weigh 1/20 of a lb. This was for a game-mechanics reason rather than for any reason of historical accuracy, but the underlying link between money, experience, encumbrance and movement speed remains intact. Coins in TH aren't decimalized either, though, so the link between weight and value becomes less obvious.