After the first installation in this series (about different movement rate systems in OSR games) the turn has come to the differing armor class systems that has been used over time.

There has basically been 2 different versions of armor class (AC) over time:

Descending AC (DAC) – In this system the AC gets better the lower the number is. To complicate things, early D&D actually used two different variants of DAC. In basic D&D (b/x, BECMI, RC, and similar variants the worst AC (represented by an unarmored person) was 9. In AD&D (1e and 2e) the worst AC was 10, so the tables get a little skewed. DAC (base 9) is used in Labyrinth Lord, Swords & Wizardry, and Old School Essentials while OSRIC uses DAC (base 10).

Ascending AC (AAC) – In third edition D&D (and onward) the AC gets better the higher the number is, but similar to DAC, the worst AC is 10 and then it increases from there. Lamentations of the Flame Princess (LotFP) uses a similar system, but the base number (unarmored person) is 12. AAC is used by Castles & Crusades, Blood & Treasure, Fantastic Heroes & Witchery and is an optional rules variant in both Old School Essentials and Swords & Wizardry.

The table below shows the different systems side-by-side and I have added some iconic armor types that are available in all systems for easy comparison.