Feathering, or Foliation
Feathering, Or Foliation an arrangement of small arcs, separated by projecting points or cusps, to ornament the inside of larger arches, or triangular or circular openings in Gothic architecture. Feathering was first introduced at the close of the early English style, and continued till the supplanting of the Gothic by the Renaissance architecture. When smaller arcs are added to ornament these small arcs, the feathering is said to be double. It is' also sometimes made triple in the latest decadence of the Gothic architecture.--Parker, Glossary of Architecture.