There are no hard-and-fast rules when writing to multiple people comprising a board of directors, which means you have several options for addressing your letter. Generally, you'll prepare one letter when the board is so large it becomes cumbersome to address each director individually. For smaller boards, you can write a separate letter for each person and use the "carbon copy" notation to reference the other recipients.

When Writing to a Large Board 

Where the board comprises five or more individuals – this number is guidance, not a rule – it is common to prepare one letter addressed to the board as a group and mailed to the company they govern. Here's an example:

The Board of Directors
ABC Property Corporation
123 City Street
San Francisco, CA 94105

Your salutation should read "Dear Board of Directors:" or "Dear Members of the Board:" followed by a colon, not a comma. If you are the business owner or another board member, it's acceptable to use the informal salutation "Dear Board" or "Dear all:"

Using a Distribution Block

Since many boards meet infrequently, perhaps once every month or so, you may wish to mail a copy of the group letter to each individual board member at their home or permanent business address. This ensures that your letter is read and able to be acted upon in advance of the next board meeting. To achieve this, write a distribution block at the end of the letter listing the name and address of each board member. Print out several copies of the letter and mail a copy to each person on the distribution list.

When Writing to a Small Board

When addressing a smaller board of directors – for example, with three or four directors – you should list each recipient by name. Your address block now looks something like this:

Ms. Robin Birch, Chairperson
Mr. Jack Haslam, Director
Dr. Olivia Blower, Director
ABC Property Corporation Board of Directors
123 City Street
San Francisco, CA 94105

Your greeting should then list the directors' names in the same order as the address block: "Dear Ms. Birch, Mr. Haslam and Dr. Blower:" It's fine to use first names if you know the recipients well and this is your usual mode of communication. Etiquette demands that you send a separate letter to each person, so print and sign an original copy of the letter and envelope for each recipient.

Using the CC Notation

Where the board members are located at different addresses, it is sensible to write a separate letter to each recipient. Here, you should use the "courtesy copy" or "carbon copy" notation to let each recipient know who else has received the letter. Do this by adding the letters "cc:" followed by the names of the other recipients at the bottom of the letter, immediately below your signature and printed name. Here's an example of the formatting:

Ms. Robin Birch, Chairperson
ABC Property Corporation Board of Directors
123 City Street
San Francisco, California 94105

Dear Ms. Birch:

[Letter]

Sincerely,

Your Name

CC: Mr. Jack Haslam, Dr. Olivia Blower

You would address your second letter to "Mr. Jack Haslam" cc'ed to "Ms. Robin Birch and Dr. Olivia Blower," and so on. It's a good idea to proofread all your copies to make sure you've included the correct recipient and courtesy copy names.