At the moment in-breeding is considered to have taken place when two animals that are very closely related are mated together i.e. brother-sister, mother-son, father-daughter. Line breeding is the mating together of two related animals, cousins, grandfather-grandaughter etc.
Glen of Imaal Terriers, in the 50s, 60s and 70s, were quite regionalised so, being mainly working animals, they were very much “horses for courses”. One area would prefer one sort of Glen because the work it did was different and obviously the landscape was different from a hundred miles away where another sort of Glen was preferred. This did lead to different types and usually type is mated to type.