Wkipedia has a pretty good explanation
http://en.wikipedia.org/wiki/Tabby_cat
“The mackerel tabby pattern has vertical, gently curving stripes on the side of the body. The stripes are narrow, and may be continuous or broken into bars and spots on the flanks and stomach. Often, an ‘M’ shape appears on the forehead. Mackerels also feature a ‘peppered’ nose, where black spots appear along the pink tip of the nose. Mackerels are also called ‘fishbone tabbies’ probably because they are named after the mackerel fish.[5] Mackerel is the most common tabby pattern.
Classic (or ‘blotched’) tabbies have a similar ‘M’ pattern on the head, but the body markings are different, having a whirled and swirled pattern with wider stripes that make what are referred to as “butterfly” patterns on their shoulders, and usually a bullseye or oyster pattern on the flank. The legs and tail are more heavily barred and the pattern is variable with respect to the width of the bands.
The ticked tabby pattern produces hairs with distinct bands of color on them, breaking up the tabby patterning into a salt-and-pepper appearance. Residual ghost striping or “barring” can often be seen on the lower legs, face and belly and sometimes at the tail tip.
The spotted tabby may not be a true pattern,[citation needed] but a modifier that breaks up the mackerel pattern so that the stripes appear as spots; the stripes of the classic pattern may be broken into larger spots. Both large spot and small spot patterns can be seen in the Australian Mist, Bengal, Egyptian Mau, and Ocicat breeds.”
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