Article RE 306
Characteristics of postage stamps and postal prepayment or franking impressions
1 Postage stamps and postal prepayment impressions
1.1 Postage stamps and postal prepayment impressions shall bear the name
of the country of origin in roman letters. They may show their postage value
in roman letters or arabic numerals. They may also bear the word "Postes"
(Postage) in roman or other letters.
1.2 They may be of any shape provided that, in principle, their vertical or
horizontal dimensions are not less than 15 mm nor more than 50 mm.
1.3 They may be distinctively marked with punched perforations or embossed
impressions produced by means of an embossing stamp, subject to the
conditions laid down by the administration which has issued them, provided
that those processes do not interfere with the clarity of the particulars
prescribed under 1.1.
1.4 Commemorative or charity postage stamps may bear an indication of the
year of issue in arabic numerals. Similarly, they may bear an inscription in
any language to indicate the occasion for which they are issued. When a
supplementary charge is payable in addition to their postage value, they
shall be so designed as to leave no doubt about that value.
Notes:
306.1.1 It is left entirely up to adms to choose the colour of postage stamps.
The text of this para allows the production of postage stamps without indication of postage value.
Article RE 309
Unpaid or underpaid items
1 The administration of origin shall be responsible for checking that international
mail items posted in its country are correctly prepaid. On arrival in the
country of destination, every item not bearing the T stamp impression in accordance
with the provisions under 8 shall be considered to be duly prepaid and treated
accordingly.
.
.
.
From UPU Publication "Letter Post Manual", pub. Berne 2001.