Writing the address of an apartment correctly when addressing an envelope helps to ensure delivery to the right location and speeds up the delivery of your mail. Timely delivery is crucial if you are mailing an invoice or other important customer communication. Generally, you will write the apartment or suite number on the same line as the street address, preceded by the abbreviation "Apt." or a hash sign.

Choose the Correct Envelope

Send standard customer letters in a white, manila or recycled paper envelope. If the communication requires extra protection, select a padded or waterproof envelope. Bills, statements of account and handwritten letters to your customer must be sent as priority mail. You can purchase priority mail envelopes at any post office or through your usual stationery supplier.

Format the Envelope

Type the apartment address on the front of the envelope in the following format:

Name of person or organization
Building number and street name, apartment or suite number
City, State and ZIP Code

Include the apartment number after the street name. You can abbreviate apartment to "Apt.", for example "32 Main Street Apt. 202." The abbreviation for Suite is "Ste." If the space available for the address is not large enough for you to write "Apt," use the hash symbol in its place: "32 Main Street #202." Do not use commas or periods. To find a ZIP code, visit the ZIP code look up service on the U.S. Postal Service (USPS) website.

While there is no requirement to type or computer-print the address label, doing so will increase legibility. The USPS online "click-n-ship" service allows you to buy stamps, request package pickups and format, print and pay for your priority mail shipping labels online. USPS will also track your domestic mail free of charge; for more details, see the USPS website. If you are writing the address by hand, print it clearly with a pen so the address is readable from an arm's length away.

Include a Return Address

Include your return business address in the upper left corner on the front of the envelope. This will enable the post office to return the letter to you in the event of non-delivery. The return address has the same elements as the delivery address. You can also mail your letter in a personalized envelope that has your business address pre-printed in the correct location. This presents a good opportunity to brand your mailings with your company name and logo.