The primary criterion of a working terrier is that it has an owner or keeper who works it. A terrier is not a working terrier by virtue of its breeding alone. The second most important quality of a working terrier is small chest size. Though the chest size of the working terrier that can be used in any given situation may rise or fall depending on the size of the den pipe, smaller dogs generally do as well as or better than larger dogs. The reason for this is rather simple: a small dog can get to the quarry without having to dig, and arrives at the quarry without fatigue.