What is Window Snap-In Glazing Bead?

What is the best source for finding window plastic glazing bead or spline? 

In need of glazing bead replacements for my old wood windows - can you help?

BEST ANSWER: Glazing bead trim pieces can be made of wood, vinyl, PVC, Plastic or aluminum that are installed around the perimeter of the window or door sash to secure the glass pane in the window sash frame. Glazing bead is normally a "snap in" type, but can also be installed in other ways - proper glazing bead should securely install the glass panels while blend with the adjacent trim pieces and set into the entire assembly. Most removal of old glazing bead will require new glazing material to be installed. The best online source for glazing bead replacements is at BiltbestWindowParts.com