This chart can be found in both OD&D Supplement I: Greyhawk and Holmes Basic, along with the modified version (basically cutting all entries for Int scores less than 9) in the 1st edition Players Handbook. I like the pre-AD&D version because, by lack of the Int requirement, it demonstrates that any idiot can become a magic-user.
I think this table clearly shows a three phase process in the development of Gygax's ideas for MU spells.
Phase 1 - Here's the spell list, kid.
In the beginning Gygax drew up his draft spell list and all MUs had access to all the same spell list. That seems to be the least tortured reading of OD&D Book I: Men & Magic. If my guy can cast third level spells, he can choose from any third level spells in the rulebook and/or research his own.
Phase 2 - Too many dang spells!
Several possible pressures led to a revision introducing the above table. Perhaps some of the players in Gygax's campaign started reaching higher levels and the sheer volume of available spells was upsetting the balance of the game. Or spell research and sharing increased the number of spells available to PCs. Or the fact that all MUs cast sleep and fireball was getting boring. Or some combination of all these factors. Either way, the result was that now everyone had to roll to see if they understood each of the spells on the official lists, with a minimum and maximum number available to each caster. Now all but the smartest MUs had a more reasonable number of spells (and like the more infamous 18/% strength, there's suddenly a new reason why high stat scores mattered).
Phase 3 - Spell as commodity
The next phase is seen in the publication of the 1st edition DMG, with its charts for starting spells. Under this system each MU starts life with Read Magic and a handful (3-5) other spells. At each level the budding magic-user may add a single spell to their book automatically. The rest they have to find within the milieu (i.e. via purchase, discovery, reaserch, etc.). Spells are also commodified in the rules in Moldvay Basic and its successors, where you start out with a single spell and spellbooks only hold as many spells as you can actually cast in a day.
Suddenly with phase 3, the Minimum column on the chart above makes a lot less sense to me. What does it mean if my first level magic-user begins play with an Int of 17 but only 4 spells in his spellbook? The chart says I should know a minimum of seven first level spells. Where are those other 3 spells? I suspect that the transition from Phase 2 to Phase 3 happened between the publication of the PHB and DMG. The Minimum column only applies to Gygax's Phase 2 thinking, which was outdated by the time he penned the starting spell charts.
Note that I think that each of these three phases makes for a perfectly acceptable spell system. Phase 3 encourages a more Vancian model, where a major activity of successful MUs will be tracking down new spells to add to their book. Phases 1 and 2 basically imply that in your setting we are all one big happy magical society; everyone who graduates from Hogwart's gets the exact same education and as they advance the Wizard's Guild doles out the standard spells. Phase 2 rewards the talented and punishes the mediocre, but the Phase 1 model is easy as pie to implement.
In some ways I really like the Phase 2 approach and that's what I've been using in my current campaign. But there's one big drag: I hate forcing the players to sit down with the spell list and roll percentile dice umpteen times just to get their 1st level MU ready to go. I want chargen to be super-fast and painless, but the Phase 2 approach is clunky as hell.
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