Beading is the thin strip of moulded timber that holds something in place around a window. The word covers more than one component, which is why window beading means different things to different people, and it is worth separating them before any repair is priced.
The glazing bead is the slender strip pinned around the edge of a pane on the inside of the sash. It holds the glass against the rebate and is the alternative to a putty-faced finish on the outside. The staff bead is the moulding fixed to the inside face of the box frame, holding the lower sash in its channel. The parting bead sits in a groove in the pulley stile and separates the two sashes so they pass each other without touching. Each is a different section, cut to a different profile, and none of them is interchangeable.
When people search for beading around windows they sometimes mean none of these: they mean the decorative timber trim where the frame meets the plaster, which is properly the architrave. On a period elevation the architrave, the apron beneath the sill and the beads inside the frame usually share a family of profiles, and matching that family is what keeps a repair from looking like a repair.
Beading in restoration
Beads take the wear in a sash window. Staff beads are prised off whenever a sash comes out, so they split at the mitres and the nail holes enlarge until the bead no longer grips. Parting beads shrink, work loose in their groove and let the sashes rattle. Glazing beads suffer wherever water has stood along the bottom rail.
Original beads are retained wherever the timber is sound, because old growth softwood is denser than anything sold today and holds a crisp profile. Where a bead is beyond saving it is replaced, and the replacement is run to match the original section rather than bought off a rack in a nearest-fit size. Getting that wrong is visible from the pavement: a modern ovolo in place of a period lamb's tongue changes the shadow line along the whole elevation.
Beads are also where draught-proofing lives. New staff and parting beads can be routed to carry a discreet seal, so the improvement is concealed inside components that were going to be replaced anyway, with nothing added to the face of the window.