A countersunk screw, also known as a “flat-head screw,” sinks into a surface and rests flush in materials. Similar to other screws, a countersunk head screw has ridges that twist into a material to provide fastening power. Countersunk screws have a flat head instead of a rounded or semi-rounded one. The Countersunk Head Screw, also known as CSK Head Screw. This kind of screw is characterized by the unique design of its head: its “flat-head” screw sinks into the surface and rests flush in materials. For this screw may countersink into the material, it's called Countersunk Head Screw.