three-quarter slab superimposed upon it, or _vice versa_. Thus are the walls in Figs. 19 and 20 built up. For openings, the jambs and lintels (and in window-openings the sill) are made solid with a provision for a key-hole to the mass of concrete filling behind them. That portion of the jambs against which the slabs butt has a groove coinciding with a similar one in the edge of the slab, for the purpose of forming a joggle joint by squeezing the bedding material into them or by joggling them in with a cement grout. All the slabs are joggled together in a similar way. [Illustration: FIG. 21.-FIG 25.] The plastic concrete filling or beton which the shells are made to contain may be deposited between the slabs when any number of courses (according to convenience) have been built up, and when set practically forms with the solid work introduced a monolith, to which the face slabs are securely keyed. With over-clayed Portland cements, which are known to contract in setting, and with those over-limed cements which expand (both of which are not true Portland cements), the filling in is done in equal sections, with a vertical space equal to each section left between them until the first sections have become thoroughly hard, and these are then filled in at a second operation. In order to provide for flues, air-passages, and ways for electric installations, and for gas and water, pipes (made of an insulating material if required) or cores of the required shape are inserted in the plastic beton, and where necessary suitable openings are provided on the face of the work. Provision is also made for fixing joinery by inserting, where required, slabs made or partly made of a material into which nails may be driven, such as concrete made with an aggregate of burnt clay, coke, and such like. Hollow lintels are also made of the slabs keyed together at their vertical joints, and when in position these are filled in with beton. This system, however, is only recommended for fire-place openings instead of arches.