Cabinetmakers have searched for a material to place inside the stile to allow for expansion and to keep the panel centered.
Carpet foam eventually loses elasticity and ceases to work. The rubber in Panalign Strips is specially designed to spring back indefinitely, keeping panels centered while allowing them to expand and contract with changing humidity. The rectangular shape makes the strips easy to handle. Typical doors take between 4-8 strips each.
They are also the solution for a panel that has been sanded so much that is loose and rattles: inserting the strips keeps the panel snug but not fixed.