Good stone walls have very few vertical joints – always two stones over one, one stone over two.
This way the vertical joints are broken as often as possible.
Joints of lime mortar do well not to exceed a certain size (1.5 cms roughly). Lime mortar can stiffen and carbonate better and it’s surface is less exposed to weather, when the joint size is kept small.
They serve to reduce the joint size where necessary and also to prevent larger stones from rocking.