OGL content from Swords and Wizardry ( https://www.d20swsrd.com/for-players/characters/armor-class/ )
Your Referee will decide whether your game is using the “Descending” armor class system where a lower AC is harder to hit, or the “Ascending” armor class system, where a higher AC is harder to hit. Numbers for the “Ascending AC” system are set off in [brackets].
Ascending AC System: For the Ascending AC system, an unarmored person is armor class . Your armor adds to your AC, so the higher your AC, the harder it is for enemies to hit you. To calculate your armor class, look at the Armor Table above, in the “Effect on AC” column. For whatever type of armor you bought, add the number shown in brackets to your base armor class of 10. That’s your new armor class.