The following are definitions of technical terms frequently used in connection with the subject of arch masonry (see Fig. 218):

Abutment - The masonry which supports an arch at either end, and which is so designed that it can resist the lateral thrust of an arch.

Arch Sheeting - That portion of an arch which lies between the ring stones.

Backing - Masonry which is placed outside of or above the ex-trados, with the sole purpose of furnishing additional weight on that portion of the arch; it is always made of an inferior quality of masonry and with the joints approximately horizontal.

Coursing Joint - A joint which runs continuously from one face of the arch to the other.

Crown - The highest part of an arch ring.

Extrados - The upper or outer surfaces of the voussoirs which compose the arch ring.

Haunch - That portion of an arch which is between the crown and the skewback; although there is no definite limitation, the term applies generally to that portion of the arch ring which is approximately half-way between the crown and the skewback.

Heading Joint - A joint between two consecutive stones in any string course. In order that the arch shall be properly bonded together, such joints are purposely made not continuous.

Intrados - The inner or lower surface of an arch. The term is frequently restricted to the line which is the intersection of the inner surface by a plane which is perpendicular to the axis of the arch.

Keystone - The voussoir which is placed at the crown of an arch.

Parapet - The wall which is usually built above the spandrel walls and above the level of the roadway.

Rise - The vertical height of the bottom of the keystone above the plane of the skewbacks.

Fig. 218. Parts of a Typical Arch.

Fig. 218. Parts of a Typical Arch.

Ring Stones - The voussoirs which form the arch ring at each end of the arch.

Skewbacks - The top course of stones on the abutments. The upper surfaces of the stones are cut at such an angle that the surfaces are approximately perpendicular to the direction of the thrust of the arch.

Soffit - The inner or lower surface of an arch.

Span - The perpendicular distance between the two springing lines of an arch.

Spandrel - The space between the extrados of an arch and the roadway. The walls above the ring stones at the ends of the arch, are called spandrel walls. The material deposited between the spandrel walls and in this spandrel space, is called the spandrel filling.

Springer - The first arch stone above a skewback.

Springing Line - The upper (and inner) edge of the line of skew-backs on an abutment.

String Course - A course of voussoirs of the same width (perpendicular to the axis of the arch), which extends from one arch face to the other.

Voussoirs - The separate stones forming an arch ring.