Head: Somewhat brachycephalic skull-type that is broad, deep, full, of moderate size, and in proportion to the rest of the body. Overall, the head is rectangular in shape and somewhat clean cut, without excess pendulous skin. The topskull is broad, being as broad as it is long (from stop to occiput). From the front, the skull appears flat between the ears. In profile, the forehead may appear slightly arched. A median furrow that begins at the stop and runs upward toward the occiput may be visible in mature dogs. This furrow is enhanced by strongly developed temporal muscles. The brows are slightly prominent. The head should never appear elongated, narrow, snipy, or resembling in any way that of a Great Dane, nor should it be coarse like that of a Bullmastiff. The head is relatively clean cut without excess skin or wrinkle.

Eyes: Moderate in size, set well apart, and open-almond to oval in shape. The eyes are set well under the brows. The eye rims are well-fitted and darkly pigmented, without any looseness, excess skin, or visible whites or haw. Absolutely no sign of entropian or ectropian. Eye color may range from hazel-brown, amber to dark brown, the darker the better. There should be sufficient bone in the surrounding orbital sockets to protect the eyes.

Ears: Somewhat small to medium in size, set high on the skull and well apart, at the highest and widest corners of the skull. The ears are dropped and v-shaped, with rounded tips. The ears are never long, overly large, or broken. The inner edge of the ears should lie close to the head when alert.

Muzzle: Of moderate length and size, well-developed, broad, deep, and full, always in proportion to the head. Upper and lower jaws have good bone substance, are strong, well-developed, and never appear snipey or weak. The muzzle never tapers, but instead ends rather bluntly, forming a right angle, as opposed to appearing rounded.

Nose: The nose is well-pigmented and black. The nostrils are well-opened.

Neck: Moderate length, powerfully-muscled with a slight arch. The neck tapers smoothly from the deeper and broader body toward the head. The neck should be dry for the most part, without excess skin or pendulous dewlap; however, skin may be just slightly looser on the throat and neck area.

Chest: Deep, broad, but never wider than deep. The brisket extends to the point of the elbows.

Body: Powerful, deep, substantial, and solid at maturity. The body should never appear racy or refined. Width at forequarters is approximately equal to the width at the hindquarters

Feet: Oval to round, compact, with well arched toes, and tough pads.

Tail: Set high on the croup, thick at the base and tapering toward the tip. Carried in accordance to the dog’s mood and energy level, but never tucked or carried up over the back. Tails are medium in length, with the tip of the last vertebrae extending to the hock joints when held down. The tail may be straight or gently curved.

Movement: In movement, the gait denotes power and strength; rear legs drive while forelegs track smoothly with good reach. The American Mastiff should move effortlessly, efficiently, and without any sign of encumbrance. The characteristics of healthy structure are evident: when moving away, the forelegs and rear pasterns should remain parallel to one another. When viewing movement from the front, the forelegs should remain parallel, with elbows and paws moving neither moving in nor out. From the rear, the back pads should be visible when the rear legs are extended. As speed increases, the forelimbs and hindlimbs will converge to the center line of gravity. From the side, the topline should remain firm and level. Good reach of movement in the front allows the forepaw to extend out in a line with the nose. The width between the forefeet when extended should be approximately equal to the width between the hindfeet when extended, indicating balance, good reach, and good drive. Dogs that exhibit any sign of breathing or locomotive difficulty shall be disqualified from the show ring.

Temperament: The American mastiff is a combination of grandeur, good nature, and gentleness. Dignity rather than gaiety. They are neither shy nor vicious. The well-trained American Mastiff is calm, controlled, and confident. Understanding, patient, and loving with their family, especially children. They are generally aloof towards strangers. A well-socialized American Mastiff is friendly yet sensitive and alert to changing situations. They are not aggressive by nature but will defend their family if necessary. They respond to threats with judicious warnings and courageous action if needed. Any unprovoked aggressive or fearful behavior toward people is incorrect for this breed. Any unprovoked aggressive or fearful behavior toward people is incorrect for this breed.