Belly

Colloquial term often used in describing a low spot in a horizontal section of sanitary drain line or vent line.  Low spots can trap water and solids leading to  drain clogs.

Rafter Tie

Where ceiling joists are not connected to the rafters at the top of the wall plate because they rest at a lower elevation, rafter ties are connected to the sides of the rafters where they rest on the top plate to provide restraint from the end thrust of the rafters...

Rafter Tail

That part of a rafter which extends beyond it bearing at the top plate of the wall past the outside face of a wall.

PEX

A type of water distribution pipe fabricated from cross-linked polyethylene.  PEX tubing is a plastic material used for both residential water supply piping systems and hydronic heating systems (using water). Advantages Include: Potentially Fewer Leaks : PEX plumbing...

Xs

(2Xs 2×4 siding) The X is short for by such as a 2 by 4 inch plank can be indicated by a 2X4 inch plank.