In the photo below, you can see a double-wythe brick wall cut right down the middle. So, what is a wythe?
Middle English from Old English withthe(“withe”)
A wythe of brick is a single vertical assembly of units. In this case, and typical with above grade exterior or even interior demising walls in Capitol Hill the wall is a double wythe.
You can see a perfect cross-section of the bedding mortar and stretcher mortar between bricks. The vertical joint between wythes is called the collar joint.
In this picture, you can see where a wall was dismantled, exposing the typical functional assembly of common brick. You can also see that this is a double-wythe brick wall. Whereas in the picture above you can see the bedding mortar between the Shiner faces of the brick, in the picture below, you can see the prepend mortar, in section exposed, between the rowlock faces of the brick.
In the photo at top, you can also see the (otherwise, normally completely-hidden) collar joint between the two wythes.