One of the fun things about the OSR is that it has inspired me to pull out the old books and try to figure out why rules were designed the way they were.
Case in point, why did D&D use gold pieces as a standard of weight? Why not use a real world standard, like pounds or kilograms?
The AD&D Player’s Handbook specifies that encumbrance is measured in Gold Pieces, with ten gold pieces equal to one GP of weight. One GP of weight is roughly equal to one pound, but it isn’t a direct comparison. The Dungeon Master’s Guide clarifies that encumbrance is not a true measure of weight, but an abstraction of weight and volume:
“Many people looking at the table will say, ‘But a scroll doesn’t weigh two pounds!’ The encumbrance figure should not be taken as the weight of the object – it is the combined weight and relative bulkiness of the item.”
-DMG, Pg. 225
This is a reasonable, if fiddly, explanation for why D&D wouldn’t simply use standard measures of weight. However the reason for the Gold Piece standard goes deeper than just being an abstraction of weight and volume, its purpose is also to re-enforce the focus that early D&D was about finding treasure. The Player’s Handbook section on Encumbrance states:
“Lastly, as the main purpose of adventuring is to bring back treasure, provision for carrying out a considerable amount of material must also be made.”
-PHB, Pg. 101
Mentzer’s Basic edition also ties the importance of treasure to the mechanic of encumbrance:
“One coin of treasure, whatever the type (gp, ep, and so forth) weighs about 1/10 pound. Since coins are the commonest of treasures, the coin (not the pound) becomes the simplest unit of weight. From now on, the weight of all treasures, equipment, and so forth will be measured in coins, abbreviated cn.”
-Basic D&D Player’s Manual, Pg. 61
Dungeons and Dragons is full of seemingly arbitrary rules, but it’s fun to dig back into half-remembered concepts and discover the method behind the madness; that they were meant to re-enforce the vision that Gygax and Arneson had for the game.