One aspect of silhouette in the Borzoi standard is underline. The standard does not specifically mention underline; however, this aspect of the breed reflects the combination of chest, ribs, back, and loin.
The standard says the following:
Chest rather narrow, with great depth of brisket. Ribs very deep, giving room for heart and lung play. Back rising a little at the loin. Loins rather tucked up, owing to the great depth of chest and comparative shortness of back and ribs.
The silhouette of the Borzoi should have smooth curves from head to toe. With a slight rise at the loin, a deep chest, and a well-muscled loin, the tuck-up should be one of the more striking transitions of Borzoi conformation.
For most of the standard there is moderation in everything but the tuck-up should be evident for both form and function. A strong loin and a deep chest enable the heart and lung capacity, combined with a strong power train, to drive the Borzoi in both the trot and the double-suspension gallop necessary in coursing.
In recent years little attention has been placed on the underline, and some Borzoi have gotten tubular in shape. That severe tuck-up is a breed element of the Borzoi, as well as many sighthounds. If the loin is too long and not well muscled, the underline does not have adequate tuck-up.
Form versus function is always a balance between power and athleticism. Flexibility is key, and the loin should be flexible, not rigid. Spring in the loin should always be from a topline that is above level to level—never below level when viewed from the side. A soft topline is not functional and often is a sign of a loin that is too long and a dog in poor condition.
Of course, compared to a dog a bitch may a have a slightly longer loin, and as they age or after a litter a bitch may have a softer topline. However, there still should be a rise and a tuck-up from that great depth of chest to the muscular loin.
Breeders need to keep this breed element in focus and not trend toward long, tubular dogs. Silhouette is extremely important in evaluating the Borzoi, and judges need to be cognizant of the tuck-up and not reward dogs who do not have this defined amalgamation of chest, ribs, back, and loin as stated in the standard. Proper evaluation is done both with the eye and the hands, and when coat obscures the dog be sure to not only check the topline but also feel for that great depth of chest and tuck-up to a well-muscled loin.
The Borzoi is series of graceful curves with easy transitions. This includes both the top of the dog and the underside of the dog all adding to that silhouette that defines this breed.
—J.T.S., Borzoi Club of America (February 2015 AKC Gazette)