What is a attention line in business letter?

The attention line of a business letter is the specific department or the name and title of the specific person to whom you are writing.

In the past, the attention line was written two spaces (approx.) below the full address; however, since the introduction of automated mail sorting equipment, placing the attention line outside of the address can delay or misdirect your letter. The best place to put an attention line is above the main address of the recipient. Examples:

ABC Company

Attn: Sales Department

123 Main Street

Anycity, etc.

OR

Dr. A.B. Farley

Research Department

ABC Company

123 Main Street

Anycity, etc.