This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
246. In joining a new wall to an old, when the walls come at right angles, the new work should not be toothed or bonded into the old work, unless the new work is laid up in cement mortar. All mason work built with lime mortar will settle somewhat, owing to a slight compression of the mortar joints, and this settlement is apt to cause a crack where old and new work is bonded together. In place of toothing, or bonding, a groove should be cut perpendicularly in the old wall, usually the width of a brick, to make a joint; this is called a "slip joint."
This method of bonding is shown in Fig. 103. At a is seen the groove or chase cut, where the new wall is to enter in the old wall; c is the new wall, and d the old wall.
In cheap construction, where new work is bonded into old, the method most commonly used is to nail a piece of 2" X 4" timber against the wall, as shown in Fig. 104, where a shows the 2" X 4" timber spiked to the old wall, and entering the center of the new wall; at b is shown the old, and at c the new wall.