Where two walls differing considerably in height come together, as for instance where the front or side wall of a church joins the tower, the two walls should not be bonded together, but the low wall should be "housed" into the other, so as to form a continuous vertical joint from bottom to top, as shown in Fig. 118.

Such a joint is called a slip joint. All mason work built with lime mortar will settle somewhat, owing to a slight compression in the joints, and this settlement is sometimes sufficient to cause a crack where a high and low wall are bonded together. In such cases there is also a chance for uneven settlement in the foundations, even when carefully proportioned. With a slip joint a moderate settlement may take place without showing on the outside.