11 RULES OF WRITING - RULE 2

RULE 2 -- Use commas to enclose nonrestrictive clauses or phrases, which are not essential to the sentence's meaning.

My window as dirty as it is reveals the beauty of nature on a snowy morning.
My window, as dirty as it is, reveals the beauty of nature on a snowy morning.

King and Lucille, his customized black Gibson have electrified audiences all over the world.
King and Lucille, his customized black Gibson, have electrified audiences all over the world.

RULE 2 -- REFERENCES

More information about this rule is available from the following sources.

Online Resources:
The Elements of Style: rule 3

Printed References: (see References section)
Strunk and White: 1-3
Hacker: P1-e