A Capped rat only has color on the head and ears, the rest of the body is completely white. There shouldn't be any other spots or specs of color, and the line between color and white should be clear and sharp.

h/h

Capped is usually caused by a modifier on the Hooded (H) locus which turns a Hooded rat into a Capped. However, it can also be due to overmarking from whitespotting genes, which can lead to megacolon, so care should be taken with the breeding of any high-white rat.

AFRMA

"Capped rats may be shown in any recognized color. The body should be a pure clean white, free from spots or brindling, with a colored head. The color should be confined to the head area only, not appearing on the throat area. The cap should be free of ragged edges or brindling, follow the line of the lower jawbone in an even line and not extend past the ears."

RSA

"The body should be a pure, clean white, free from spots or brindling, with a colored head. The color should be confined to the head area only, not appearing on the throat. The cap should be free of ragged edges or brindling, follow the line of the lower jawbone in an even line and not extend past the ears. The color shall conform to a recognized color."

RatsPacNW

"Cap on head not to go past the ears or throat.  Should have notch between ears.  Pure white body, no blaze or head spot."

URS

"The head and ears of the rat to be of any standardized color, including the underside of the jaw and chin, while the shoulders, body, feet, belly and tail should be completely white with no spots or colored hairs. The line of demarcation between color and white should be as even and sharp as possible."