To be eligible for mailing at the postage rates for letters, a piece must be:
- Rectangular
- At least 3-1/2 inches high x 5 inches long x .007 inch thick
- No more than 6-1/8 inches high x 11-1/2 inches long x 1/4 inch thick
- Aspect ratio – The aspect ratio (length of the mailpiece divided by height) must be between 1.3 and 2.5
