A king stud is a full-height stud — running from the bottom plate to the top plate — placed at each side of a rough opening for a door or window. It stands next to the jack stud (trimmer), a shorter stud that supports the end of the header. The king stud is nailed to the jack and to the end of the header, locking the assembly together and stiffening the wall against wind pushing on the opening.
A complete opening is framed with several stud types: king studs on the outside, jack studs under the header, the header itself, cripple studs (short studs) filling in above the header and below a window’s rough sill, and the sill plate across the bottom of a window opening. For exterior walls, IRC Table R602.7.5 sets the minimum number of full-height studs at each end of a header, based on the opening width and design wind speed — wider openings in high-wind areas need two or more.
For estimating, each opening typically adds about 3–4 studs beyond the regular 16” on-center layout: one king and one jack per side, minus the layout studs the opening removes, plus cripples. That is why a stud takeoff counts openings separately from the simple “length ÷ 16 + 1” layout count.