Dictionary.net

• The stones or arches thrown across the angles of a square tower to support the alternate sides of the octagonal spire: also, the cross pieces of timber across the angles to give strength and firmness to a frame.

1899 - The american dictionary of the english language.
By Daniel Lyons

• Stones or arches across angles of square tower supporting alternate sides of octagonal spire.

1919 - The concise Oxford dictionary of current English
By Sir Augustus Henry