[RokeIdeadioke]

As an absolute rule I only apply it to dog breeds that have been selectively bred in the past to have aggression toward other canines.

This doesn't mean there aren't really good reasons for keeping other breeds separated and confined. You don't want your house destroyed? You don't want an unexpected litter? etc etc. Of course this holds for just a single dog too.

However, unless I sense there is a problem, I'd not worry about leaving pretty much any other dog breeds together outside in a high fenced yard.

Remember that dogs derive from wolves, which are a pack animal and the pack is extremely necessary for survival. However, there will still be the occasional show-down over dominance...and a wolf from outside the pack that comes onto pack territory will often be killed.

Dogs still have that same pack need. In fact dogs have been bred to like humans a bit more than other dogs, but it is still all about the pack so a pair of dogs normally get along great.

Dogs derived from the pit fighters come from a group that was bred to be aggressive toward other canines. They are actually bred also to strongly like humans. A pit pup prefers human company to dog company generally a week or two before most other dog breeds. (Although I don't know if this is because they have higher LIKE of humans or because of the DISLIKE of other dogs starts kicking in) This is why two pits that have been fine alone together can suddenly switch and end up with one dead.

ALWAYS keep pits separated when not under your control. Other breeds, judge by the individuals. I've seen some ill-bred cocker spaniels that I'd not trust to be alone together, but it's more the case of I've seen other pairs and triples that I'd never dream of not leaving alone together.