how do I address a letter to a board of directors?
To the Board of Directors:
I'm not sure if you are talking about the address on the envelope and the inside address, or the salutation. If you are talking about the former, it would be:

Board of Directors

XYZ Corporation (the name of whatever entity they serve on the board of)

street address

city, state, mail code

If you are talking about the salutation:

If the board is made up of both men and women, then the standard is,

Dear Ladies and Gentlemen:

However, no one would fault you if you wrote:

Dear Board Members:

To the Board of Directors should only be used in memo form.
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I was indeed referring to both; Thank you.