There are three armor types in classic D&D (B/X, BECMI, Cyclopedia). Leather (AC7), Chain (AC5), Plate (AC3).  In terms of classes, fighters, elves, dwarves, halflings and clerics can use all armor, thieves only leather armor and magic-users no armor. This is important, because this frames why people wear the armor they do. Why would anyone wear leather armor? Because they can. That means thieves. Thieves can wear leather, magic-users can not and that gives thieves an advantage. It's also cheap and only weights half what chain weighs. There are no real downsides to wearing light armor over no armor, except weight. There is rarely much reason to wear chain armor though. Plate mail is only slightly heavier and pricier and any class able to wear chain can also wear plate. In other words, chain is almost never a meaningful choice. It's going to be all Clyde Caldwell for this entry. It's All AC5, baby. So maybe we should find some reasons to make the choice between chain and...