Whilst fighting ability and thieving ability are barely attested and at best ambiguous terms in the first edition Player’s Handbook and Dungeon Master’s Guide, spell ability frequently appears and is fully defined in the former work as indicating "whether or not the class of character is able to employ spells" (p. 19). It is further divided into four types, which is to say magic-user, clerical, illusionist and druidic, each corresponding to one of the four major spell casting classes. However, much like the fighting and thieving characteristics, spell ability also has a narrower definition, referred to in the Dungeon Master’s Guide entry for the Ring of Wizardry; it is explained there that the "ring doubles spell ability (i.e. the number of spells a magic-user may prepare each day) in one or more spell levels" (p. 132). As with the other entries in this series of articles, it is the more limited definition that is of interest here, spell ability as an indication of the number of spell slots available to a character by level. Whereas the spell slot progressions in the various iterations of Dungeons & Dragons vary considerably in structure by class, the contention here is that a more consistent approach would be useful and do no violence to the overall system.
In the original Dungeons & Dragons game there were initially two spell casting classes available, the magic-user and the cleric, with the illusionist and druid subclasses being later additions in Strategic Review and Eldritch Wizardry, respectively. Famously, at first level a cleric had no spell slots available, his progression only beginning at second level, but what is rarely noted is that at eleventh level the magic-user and cleric have exactly the same number of spell slots available and this remains the case at twelfth level when they both first get access to spells of the sixth level. After that they deviate again, partly because their maximum spell levels differ (level nine for magic-users and level seven for clerics). The spell progressions for classes are slightly different in each subsequent edition, including the D20 version of Dungeons & Dragons, and this interesting transient equality is lost. On the other hand, in the advanced game, magicians and clerics nominally have the same number of slots available from levels one to four, but in practice any cleric with a wisdom score of thirteen and above has rather more. Their subclasses, the illusionist and the druid, follow completely different progressions of their own.
Apart from the major spell casting classes and their multiclass combinations, there are the fighter subclasses with minor spell casting ability, which is to say the paladin and the ranger, and not counting the capacity of the thief class to read scrolls. Of the two subclasses, only the ranger originally had any spell casting ability, though the paladin as he appeared in Greyhawk could always "lay on hands", "detect evil", and "dispel evil", which applied to "spells, undead, evil enchanted monsters, and the like" (p. 8). Indeed, it was only in the advanced game that the paladin acquired the ability to cast spells at relatively high levels, along with the precondition in the Dungeon Master’s Guide that such characters had served a "novitiate" much like the cleric and druid (p. 39). Along with the details in the same section (pp. 38-40), this reflects an increasingly complex and specific approach to explaining how magic works in Advanced Dungeons & Dragons and as a result also partially defining its limits. The positive and negative energy planes have a central role to play in the process, always triggering the channelling of energy from other planes of existence or else serving as the prime source themselves, the effects of which then manifests of the prime material plane.
Perhaps the most significant delineation in the advanced magic system is the acquisition, memorisation and preparation of spells for clerics and magicians. For the latter, a spell may be taught by an already learned master, studied from a book or researched anew, but in all cases the magician is limited in both the number he may ever learn and even which spells he has the aptitude to learn. After successfully acquiring a spell it must be maintained in a spell book and memorised from the text whenever it is to be prepared. The cleric, by contrast, has knowledge of all existing spells for his class, but in order to prepare those of third to fifth level must communicate with intermediaries of his deity and have them bestow the spells requested upon him, whilst for spells of sixth to seventh level must petition the deity directly. Nonetheless, in order to prepare spells of the first and second level the cleric need only rely on his training and faithful service to his deity, which is to say the spells he desires are bestowed without direct communication, apparently simply a matter of prayer and meditation. There is something vaguely conceptually dissatisfying about this treatment and the open ended character of the clerical spell list is potentially troublesome.
Limiting the number of spells available to the cleric is most easily done by imitating the lot of the magician, requiring each spell to be acquired individually and maintained in a "prayer book", which is how it currently works in my Silver Blade and Greyhawk campaigns. When using published modules the simple expedient of treating spells memorised as spells known for non-player characters and monsters has proven to be largely satisfactory. Conceptually, the cleric is reimagined as a holy warrior so worthy in his abilities and faithful in his character that a deity or pantheon has empowered him with spell ability. As he rises in ability level the cleric must petition, usually through otherworldly intermediaries, for access to higher spell levels. Whilst the magician seeks arcane knowledge and may traffic with extraplanar beings to obtain it, understanding for the cleric comes in the form of divine revelations as to the nature of the multiverse, often conveyed by the very same outside intelligences. Regardless, not everybody, indeed few, have the potential to become clerics or magicians, whether it is a matter of inner qualities, external selection, diabolical compacts or a mixture of some or all, few can speculate with authority and none can say for sure.
There is no real reason for magicians and clerics to have different spell progressions, excepting perhaps an appreciation for an eccentric and erratic aesthetic, not lightly discounted by all. Still, if levels one to four and levels eleven to twelve can be the same, why not reasonably levels five to ten? As a known and stable value spell ability might be easier to design around and certainly easier to notate in statistic strings, though "SA" might cause confusion with "special attacks" or "special abilities", so might be more productively rendered "SCA". That said, acronyms could be created for the already existing spell ability divisions by class and subclass, if that were preferred, for instance "MSA", "CSA", "ISA", and "DSA". Either way the use of spell ability as describing the number of spell slots available by level is potentially useful. For those interested, a comparison of spell ability across the extant editions and classes of the game can be downloaded here in portable format document, including for Swords & Wizardry, Labyrinth Lord, and the Old School Reference & Index Compilation. Differences from the original game are highlighted in red, from the basic and expert game in blue and from the advanced game in green.