FR in FR4 stands for flame retardant, whereas the number 4 differentiates the material from other materials in this class. FR4 is a fiberglass-reinforced epoxy laminated sheet, which looks like a thin, and woven cloth sheet. The term FR4 also stands for the grade used to make these laminated sheets. The fiberglass construction provides structural stability to the material. The fiberglass layer is covered with an epoxy resin that is flame-resistant. This lends durability and strong mechanical properties to the material.