Many modern doors, including most interior doors, are flush doors: Core material: Material within the door used simply to fill space, provide rigidity and reduce druminess.
Hollow-core - Often consists of a lattice or honeycomb made of corrugated cardboard, or thin wooden slats. Can also be built with staggered wooden blocks. Hollow-core flush doors are commonly used as interior doors. Lock block - A solid block of wood mounted within a hollow-core flush door near the bolt to provide a solid and stable location for mounting the door's hardware.