BOB. Big on Big; Back on Backer. This is a man blocking scheme that is optimal for many teams because of its ability to get a hat on a hat, an offensive lineman on a defensive lineman, and a back on a preferred LB (if a back is in to block).
First let's look at a world where 5 person blocks exist. Whoever is the uncovered lineman is will be the "double read" blocker. The OL always wants to work to protect against the most immediate threat first. Who that is may depend on defensive tendencies or personal preference, but we're not going to get into that in great depth in this article.
The Big on Big concept will always work to "half-man advantage", that is, a non-center offensive lineman will block a defensive lineman that is lined up directly over top or shaded over him (half-man). Whoever is the bubble offensive lineman (doesn't have anyone over him or shaded outside of him at the snap) will work with the person inside of him (typically an OG and center) to the MIKE (MIKE as defined by the offense, not necessarily the MLB). The center doesn't necessarily follow the half-man concept, he will make a right or left call and that will dictate the blocking scheme
Notice here that the OG and the OC can work together to pick up the NT and either the MLB or the WLB. In a 5-man blocking scheme, this is a 2-on-3 blocking scheme that can pick up the entire DL and one interior blitzer. It is important to note though that if both LBs blitz, they can't both be picked up, and must be accounted for with a hot read.