Back and Underline
Arched Over the Loins
The term "arched over the loins" refers generally to a back that is level and arches to a greater or lesser extent over the groin area. This arch is usually due to muscle development, hence denoting the strength from this area. Rhodesian Ridgebacks and Dachshunds are a couple examples of breeds that require this arch.
The term "level back" means that the height at the withers is technically identical to that of the height at the loins.
A long back is considered to be a breed in which the distance from the withers to the rump is appreciably longer than the height of the dog at the withers. Ex. Dachshunds or Corgi breeds.
The roach back is considered a fault for the vast majority of breeds, but some breeds do require it. A roach back is slight arch over the groin. For this reason it is important to make sure that the arch is correct or the result of some faulty construction.
A dog with a sloping back means that the height at the withers is higher than at the loins, such as the German Shepherd. Though technically it can also mean that the height at the loin is higher than at the withers, such as the Old English Sheepdog or the Chesapeake Bay Retriever.
Straight back doesn't necessarily mean a level one. It simply means that the back is completely straight, with no dip or arch, between the withers and loin. This type of back can be seen in the English Toy Terrier.
A wheel back is a form of the roach back that is exaggerated. It is a continuous arch that extends from the withers to the loin. The Borzoi and Bedlington Terrier are two types of breeds that have this type of back.
The tuck up, or cut up as it is sometimes called, is the shape that is produced by the underline of the abdomen as it sweeps up to the region of the hind quarters. It can be exaggerated in some breeds, such as the Whippet, and is further exaggerated when they have a deep chest. It can be barely moderate or even barely noticeable in other breeds.