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.