Nine- by 12-inch envelopes come in padded and unpadded varieties.

When you have important documents, papers or other materials that need to be mailed, a 9-inch by 12-inch envelope is an option. Nine- by 12-inch envelopes are a good choice when the documents you're sending cannot be folded in order to be placed in a business envelope or if, when folded, the documents are still too large for a regular-sized envelope. In order to make sure your envelope arrives safely to its recipient, it's important to make sure you meet postal requirements.

Determine which type of 9-inch by 12-inch envelope is appropriate for you. If you're mailing something that should not be bent or may be fragile, a padded envelope would be best.

An overstuffed envelope may end up ripping.

Place your document or item in the envelope and ensure the filled envelope isn't more than 3/4-inch thick, as required by the post office.

Address your 9-inch by 12-inch envelope by placing the mailing address in the center and the return address in the upper left corner on the same side as the mailing address.

Bring the 9-inch by 12-inch envelope to the post office so it can be weighed on a postal weight scale to determine postage needed.