The term 'ARCH PART' is composed of two crossword solutions, KEYSTONE and RADIAL. A KEYSTONE is a wedge-shaped stone at the apex of an arch that locks the other stones into place. It is the central piece of an arch and is often the last piece to be placed. RADIAL refers to the arrangement of stones in an arch, which are placed in a curved pattern radiating from the center. This type of construction is used to create strong and stable structures.