"Which" is used to indicate an attribute that is associated with or defines a subject or object, and may be set off with commas.

"That" narrows down the selection of subjects or objects to which the sentence refers and generally does not use a comma.

Example 1:
Shabby chic furniture, which may be left with partially-peeling paint, was used to furnish the little rustic cabin by the pond.

In the group of vintage chairs in the workshop, please sand the ones that have peeling paint or faded stain.

Example 2:
First, we use "which" to associate an attribute to a particular group of cats. The tortoiseshell coloring in cats is defined by its combination of mixed orange and black fur. Second, we use "that" to identify a specific X chromosome from the cat's father.

The tortoiseshell cat, which has a combination of orange and black fur, is a representative example of X-inactivation in the female cat's two X chromosomes. (A female cat possesses two X chromosomes, only one of which is left fully activated in each of the cat's cells. Black fur coded by the mother's active X chromosome grows where the father's X chromosome that codes for orange fur is largely inactivated. Orange fur grows due to the active X chromosome inherited from the father where the X chromosome containing the gene for black fur from the mother is mostly inactivated.)

Given a room full of cats, my attention is likely drawn to the ones that have the calico or tortoiseshell coloring.

Mimi, a tortoiseshell cat

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