Plates are the horizontal members that hold a stud wall together. The bottom plate (sole plate) runs along the floor and is nailed to the bottom of every stud; the top plate caps the studs. Most load-bearing walls get a double top plate — a second layer on top of the first — which stiffens the wall, ties intersecting walls together at the corners, and spreads the load from joists or rafters that don’t land directly over a stud.
The IRC (R602.3.2) requires the end joints in the two layers of a double top plate to be offset at least 24”, and the top layer laps over at corners and wall intersections. With a single bottom plate and double top plate, a wall built from 92-5/8” precut studs stands 92-5/8” + 3 × 1-1/2” = 97-1/8” tall, which leaves room for drywall on an 8 ft ceiling. For estimating, plan on three times the wall length in plate lumber.
Where a wall sits directly on concrete, the plate must be pressure-treated or naturally decay-resistant wood (R317.1), because concrete wicks moisture. On a foundation, that sill plate is held down with 1/2” anchor bolts spaced no more than 6 ft apart, embedded at least 7” in the concrete, with a bolt within 12” of each end of every plate section (R403.1.6). Leave enough plate for the birdsmouth seat of the rafters to bear on at the top of exterior walls.