(ii) The following specific items when sent to or from schools, colleges, universities, or public libraries, and to or from nonprofit religious, educational, scientific, philanthropic, agricultural, labor, veterans, or fraternal organizations or associations; may be mailed at the Library Rate:

filmstrips;

(a) 16-millimeter films; transparencies; slides; microfilms; all of which must be positive prints in final form for viewing.

(b) Sound recordings.

(c) Scientific or mathematical kits, instruments, or other devices.

(d) Catalogs of the materials in clauses (a), (b), and (c) of this subdivision having 24 or more pages, at least 22 of which are printed, and guides or scripts prepared solely for use with such materials.

(b) Application of rates. (1) The rates in § 135.1 (a) and (b) are applied on the basis of weight of the individual

piece and the zone (or distance) to which mailed. Articles addressed to military post offices overseas (Army, Air Force, Fleet post offices, and naval vessels) require postage at the zone rate applicable between mailing office and post office shown in the address.

(2) There are eight parcel post zones, determined as follows:

(i) The United States and its territories and possessions are divided into units of area 30 minutes square, identical with a quarter of the area formed by the intersecting parallels of latitude and meridians of longitude. Each unit of area is designated by a number.

(ii) The zones are based on the straight line distance between the unit of area in which the mailing post office is located and the unit of area in which the post office of address is located, measured from the center of one unit to the nearest point in the other. The zone equiva

lents of straight line distances are shown in the column headings of the rate chart in § 135.1(a).

(iii) A zone key, prepared for each unit of area, lists all the other units of area in numerical order and indicates the zone distance to each of these other units of area. Each numbered unit of area has its own identically numbered zone key, and the key is used at all post offices in that unit of area.

(iv) To ascertain the parcel post zone (distance) to a particular post office, consult the Directory of Post Offices. Under the State listing locate the name of the post office to which the parcel is to be mailed. At the right of the name is the number of the unit of area. Then refer to the zone key of the unit of area of the Office of Mailing. This key will list the unit of area of the office of address and to the right thereof the parcel post zone. This procedure is also used to determine zones within Puerto Rico, and the Virgin Islands.

Determine the parcel post zone from Paola, Kansas (Unit 2821), to Flat Rock, Illinois (Unit 2121), using the Directory of Post Offices and the Official Parcel Post Zone Key (Unit 2821).

(v) A Directory of Post Offices may be purchased from the Superintendent of Documents, Government Printing Office, Washington, D.C., 20402. A parcel post zone key for the unit of area in which the mailing post office is located may be obtained free by request to the postmaster.

(vi) To assist mailers of large volumes subject to zone postage charges, "Parcel Post Zone Guides, ZG Series," have been prepared for selected units of area.

A guide shows the proper parcel post zones from a unit of area to other post offices in the United States and possessions. Patrons desiring a guide for

mailing at any post office in any of the units of area listed below should direct a request to the postmaster of that post office, or to the Headquarters Services Division, Post Office Department, Washington, D.C., 20260. The guides are free, and a schedule of the postal rates in § 135.1 (a) and (b) accompanies each guide, thereby enabling mailers to ac

§ 135.3 Weight and size limits.

(a) Weight. Each addressed piece must weight 16 ounces or more but not in excess of the following limits:

(1) Parcels mailed at or to any second-, third-, or fourth-class post office; to or from any rural or star route; to or from any Army-Air Force or Fleet post office; to or from any post office in Alaska or Hawaii; or to, from, or between any territory or possession of the United States, including the Canal Zone and Trust Territory of the Pacific Islands, must not exceed 70 pounds. See Part 127 of this chapter for weight limits to certain APO's.

(2) Parcels mailed at a first-class post office in the United States for delivery at the same or any other first-class post office in the United States are limited to 40 pounds when addressed for delivery in the local, first, and second zones; and to 20 pounds when addressed to the third through the eighth zones; except that parcels mailed on or addressed for delivery on a rural or star route, parcels mailed to or from any post office in Alaska or Hawaii, parcels mailed at the post office from which served by patrons

located on rural or star routes (rural or star route address of sender must be shown), or parcels containing baby poultry, nursery stock, agricultural commodities, books, other items listed in § 135.1 (c) and (d), and Braille-writers and other appliances for the blind are subject to the limit in subparagraph (1) of this paragraph. (The term "agricultural commodities" includes any product grown or produced incident to an agricultural activity on a farm or in a garden, orchard, nursery, or forest but does not include manufactured products of such commodities.) Parcels containing such articles must be marked to show the nature of the contents, unless such information can be ascertained by outward examination of the parcel.

(b) Size. Parcels mailed at a firstclass post office in the United States, for delivery at the same or any other firstclass post office in the United States, and subject to the limits of weight shown in paragraph (a) (2) of this section must not exceed 72 inches length and girth combined. All other parcels are limited to 100 inches in length and girth combined. To compute the size of a parcel:

(2) See § 121.5 § 135.1(b) (1) (i) and (2) (ii), § 135.2 (a) (4) and (5), and § 138.5 (b) for special markings required. (b) Permissible. (1) The following written additions may be placed on the wrapper, on a tag or label attached to the outside of the parcel (see § 123.2(b) of this chapter for space requirements), or inside the parcel, either attached to an article or loose:

(i) Marks, numbers, name, or letters descriptive of contents.