In the United States and Canada a special method of differentiating rates of postage has been adopted. All postal traffic is termed "mail matter," and is classified in four groups, to each of which is applied an appropriate rate. The classification, which is almost identical in the two countries, is based partly on the general character of the packets (size, shape, etc.), but more largely on certain general principles of administration, and on the intrinsic value of the contents. Thus the ordinary letter, which is the most important and valuable traffic, is placed in the first class of mail matter, and is charged the highest rate. Newspapers and periodicals, which are regarded as of great importance in aiding the education and enlightenment of the people, are placed in the second class of mail matter and are given the lowest rate.[527] Books and all other printed matter, commercial papers, postcards, etc., are regarded as of less importance than letters, and are deemed to be less entitled to encouragement from the State in their distribution, but still entitled to preferential treatment as compared with packets containing miscellaneous articles. They are accordingly placed in the third class of mail matter, and are given a rate intermediate between that of the first class and that of the second. All other articles sent by post--the residuum of postal packets--are placed together in a fourth class of mail matter, to which is applied a rate higher than the third-class rate, but considerably lower than the first-class rate.[528] The rates for first-class matter (letters) and second-class matter (newspapers and periodicals) in the United States have been dealt with. They may be repeated here for purposes of comparison: the rate for letters is 1 cent for each 2 ounces or fraction of 2 ounces; the rate for newspapers is 1 cent a pound or fraction thereof when sent from publisher to subscriber--when sent otherwise the rate is 1 cent for every 4 ounces. On third-class matter the rate is 1 cent for each 2 ounces or fraction thereof, and on fourth-class matter the rate is 1 cent for every ounce or fraction of an ounce. With the view of encouraging agriculture, seeds, cuttings, bulbs, scions, roots, and plants are given the same rate as ordinary printed matter in the third class.