United States Postal Regulations
First Class Mail Matter-Letters.-This class includes letters, and anything of which the postmaster cannot ascertain the contents without destroying the wrapper, or anything unsealed which may be wholly or partly in writing--except manuscript for publication accompanied by proof sheets. Postage, two cents each one ounce or for each fraction above one ounce. On local or drop letters, at free delivery offices, two cents. At offices where no free delivery by carriers, one cent.
Second Class - Regular Publications.- This class includes all newspapers, periodicals, or matter exclusively in print and regularly issued at stated periods from a known office of publication or news agency. Postage, one cent a pound or fraction thereof.
Third Class - Miscellaneous Printed Matter. - Transient newspapers and periodicals, one cent for each four ounces or fraction thereof. Mailable matter of third class includes printed books, circulars and other matter wholly in print (not of the second class), proof sheets and manuscript accompanying the same, and postage shall be paid at the rate of one cent for each two ounces or fraction thereof, and shall fully be prepaid by postage stamps affixed to said matter.
All packages of matter of the third class must be so wrapped or enveloped that their contents may be readily and thoroughly examined by postmasters without destroying the wrappers.
From Houghtaling's Handbook