Today's lesson is about the leathers and tannins used by clothiers when making Medium Armor. As you have noticed while hiding from the nightwatch in your father's armory, medium leathers come in many kinds: for example, brigantines are hard and studded, while running leathers are tough but flexible. These different style are achieved by the use of different hides and tannins.
Tannins are "tempers" used by the crafter to balance hardness and resiliency in medium armor. A leather chestpiece can be treated to be as rigid as a board, able to turn an assassin's knife—but if it is too rigid, the wearer is unable to turn his or her body. Thus, armorers use tannins on leathers and fabrics to strike a balance between stiffness and flexibility.
What does this have to do with the look of the resulting armor set? If only you had asked that question at lessons rather than daydreaming about impressing the scullery maid with that wisp of a mustache. It has everything to do with it, as you'll see for yourself. Now get to work on curing those hides your father bought as samples—and don't let me see you making eyes at the scullery maid until you're done.