Herewith an adaptation for Classic D&D of the race/class combination in OD&D. To handle hit dice advancement in a way which is not as clunky as in AD&D, I devised a method based on the elf's advancement in Holmes Basic; though it's not clear what's the principle behind it in Holmes, the way I abstracted it seems coherent from the point of view of advancement.
OD&D-style race/class combination
by Antonio EleuteriAll multiclass combinations increase their HD based on the level of the class with the highest original HD type (in all the cases here it’s the Fighter class, though the DM might allow different combinations). Experience points are always split evenly between all the classes even when a class reaches its maximum.
Dwarf
- Minimum Constitution 9
- +4 to all save categories except dragon breath (+2)
- Other racial abilities and limitations per Basic Players Book
- Fighter 12th level (14 with Str 17, 16 with Str 18)
- Thief unlimited level
- Fighter 12th/thief (d6 HD, leather armour only for thief functions)
- Fighter/cleric 14th level (d6 HD, as NPC only)
Elf
- Minimum Intelligence 9
- Other racial abilities per Basic Players Book
- Fighter 8th level (10 with Str 17, 12 with Str 18)
- Magic-user 16th level (18 with Int 18)
- Thief unlimited level
- Fighter/magic-user (d6 HD)
- Fighter/magic-user/thief (d6 HD, leather armour only for thief functions)
- Fighter/magic-user 12th/cleric 12th (d6 HD, as NPC only)
Halfling
- Minimum Constitution 9, Dexterity 9
- +4 to all save categories except dragon breath (+2)
- Other racial abilities and limitations per Basic Players Book
- 8th level fighter (d6 HD)
- Thief unlimited level
Half-Elf (optional race)
- Same racial abilities as elf, but is not immune to ghoul paralysis
- Fighter 12th level (14 with Str 17, 16 with Str 18)
- Magic-user 12th level (14 with Int 17, 16 with Int 18)
- Thief unlimited level
- Fighter/magic-user (d6 HD)
- Fighter/magic-user/cleric 8th (d6 HD, Wis 13 minimum Neutral or Lawful alignment)
Racial Bonuses to Demihumans as Thieves:
Dwarf: OL 5%, F/RT 15%, MS 5%, HS 5%
Elf: PP 5%, MS 10%, HS 15%
Halfling: OL 10%, F/RT 5%, PP 5%, MS 10%, HS 10%, HN +1/+10%
Half-Elf: PP 10%, HS 5%