Breed standards call out for a curled tail. While the amount of curling will vary from dog to dog and can improve as a Pug matures, this is a defining physical feature.
The breed standards regarding the Pug's tail from all major organizations in the world are as follows:
AKC (American Kennel Club) and CKC (Canadian Kennel Club):
The tail is curled as tightly as possible over the hip. The double curl is perfection.
KC (The Kennel Club of the UK), FCI (Fédération Cynologique Internationale - Largest breed registry in the world with 80+ member countries) and ANKC (Australian National Kennel Council):
High-set, tightly curled over hip. Double curl highly desirable.
As you can see, both descriptions are very similar. In essence, the tail should curl tightly and if the tail has a double curl, this is judged as more favorable in the show ring. Most Pugs have a tail that not only curls, but lies over the back. This is an element of the breed standard and is the desired look. With this said, there are some Pugs that have a loose tail. We'll dive into this ahead.