This is an exerpt as it pertains to 'Flats'. I suggest you save the web address as it defines all the physical characteristics of 1st Class Mail letters, flats, and packages. It's really quite simple really, presuming, or course, you've got an advanced degree or 25 years experience in Quality Control Testing and Measurement.

2.0 Physical Standards for Flats

2.1 General Definition

Flat-size mail is:

a. More than 11-1/2 inches long, or more than 6-1/8 inches high, or more than 1/4 inch thick.

b. Not more than 15 inches long, or more than 12 inches high, or more than 3/4 inch thick.

c. Flexible (see 2.2)

d. Rectangular

e. Uniformly thick (see 2.3)

f. Unwrapped, sleeved, wrapped, or enveloped.

2.2 Minimum Flexibility Criteria for Flat-Size Pieces

Flat-size pieces must be flexible. Boxes—with or without hinges, gaps, or breaks that allow the piece to bend—are not flats. Tight envelopes or
wrappers that are filled with one or more boxes are not flats. Test flats as follows:

a. All flats (see Exhibit 2.2a):

1. Place the piece with the length parallel to the edge of a flat surface and extend the piece halfway off the surface.

2. Press down on the piece at a point 1 inch from the outer edge, in the center of the piece's length, exerting steady pressure.

3. The piece is not flexible if it cannot bend at least 1 inch vertically without being damaged.

4. The piece is flexible if it can bend at least 1 inch vertically without being damaged and it does not contain a rigid insert. No further testing is necessary.

5. Test the piece according to 2.2b or 2.2c below if it can bend at least 1 inch vertically without being damaged and it contains a rigid insert.

Exhibit 2.2a Flexibility Test—All Flats

b. Flats 10 inches or longer that pass the test in 2.2a and contain a rigid insert (see Exhibit 2.2b):

1. Place the piece with the length perpendicular to the edge of a flat surface and extend the piece 5 inches off the surface.

2. Press down on the piece at a point 1 inch from the outer edge, in the center of the piece's width, exerting steady pressure.

3. Turn the piece around and repeat steps 1 and 2. The piece is flexible if both ends can bend at least 2 inches vertically without being damaged.

Exhibit 2.2b Flexibility Test—Flats 10 Inches or Longer

c. Flats less than 10 inches long that pass the test in 2.2a and contain a rigid insert (see Exhibit 2.2c):

1. Place the piece with the length perpendicular to the edge of a flat surface and extend the piece one-half of its length off the surface.

2. Press down on the piece at a point 1 inch from the outer edge, in the center of the piece's width, exerting steady pressure.

3. Turn the piece around and repeat steps 1 and 2. The piece is flexible if both ends can bend at least 1 inch vertically without being damaged.

Exhibit 2.2c Flexibility Test—Flats Less Than 10 Inches Long

2.3 Uniform Thickness
Flat-size mailpieces must be uniformly thick so that any bumps, protrusions, or other irregularities do not cause more than 1/4-inch variance in thickness. Exclude the outside edges of a mailpiece (1 inch from the edge) when determining variance in thickness. Mailers must secure nonpaper contents to prevent shifting of more than 2 inches within the mailpiece.

2.4 Flat-Size Pieces Not Eligible for Flat-Size Rates
Mailpieces that do not meet the standards in 2.1 through 2.3 are not eligible for flat-size rates and must pay applicable parcel rates.