Flat Size

Flat size pieces are pieces that have one dimension larger than the minimum requirements for letter size pieces (height greater than 6.125 and length greater than 11.5).  Though

Flats are mostly pieces inserted into 9x12 envelopes, full page advertisements, large booklets, etc.  Postage for flats is considerably more expensive, but flats are also the only size of mail eligible for Every Door Direct Mail (EDDM) which offers highly discounted rates.