End view of garden bed walls showing "keystone" cut to meet out of course bond

 

Because the front of the garden bed wall is lower by less than a full course a "keystone" has been cut to connect the front wall to the rear.  This provides a better bond than having a vertical break in the wall and relying on steel ties.  An old stonemasons saying is "one on two on one" meaning to avoid vertical joints in stonework going more than 1 course.