(Re posted here from one of my old defunct blogs because its still true)
One of the key differences between OD&D and 3.5+ is the way the game handles Armor Class (AC) and Attack Rolls. Early Editions of Dungeons and Dragons (LBB, Supplements, Holmes, BX, BECMI, and Cyclopedia) make use of a simple chart based system for resolving attack rolls. You find the attackers row on the relevant chart, slide over to the defenders armor class, and you’ve got your target number (To-Hit) for the attack roll. Later editions of D&D (3, 3.5, 4, Pathfinder) utilize Ascending AC where the AC is replaced by the first row of To-Hit numbers on the Attack Table such that a first level PC’s To-Hit number is identical to the targets AC. The attack roll has to be ≥ the To-Hit number to succeed, Swords and Wizardry offers both systems.