The bay locus is called "agouti" so if you read this term, it is referring to the bay gene.

Bay IS NOT a base colour... a bay looking horse is actually a black horse that has the agouti gene.  This gene lightens the body hair of black horses, but has no effect on red hair.

Bay (agouti) restricts the black pigment to the "points" of the horse, or the lower legs, mane and tail. Bay horses generally have black ear tips also.
It is a dominant colour, meaning a bay horse must have received a bay gene from at least one parent.

There are many different terms used when describing the different shades of Bay. 
Regardless of the terms used to classify a shade of Bay, all Bay horses will have a red body, ranging from light to dark, with black legs from the hoof to the knee and hock and sometimes extending above the knee and hock. The mane and tail will also be black.

Because the bay gene only effects black hair, chestnut horses that carry the bay gene do not show it, but can pass it on to offspring.  

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