The Cape Colony joined the Universal Postal Union in 1895. As a member was bound to observe the regulations applicable to members. These included the provision that all members would forward mail, even if such mail was not prepaid or underpaid. The postal authorities of the country receiving the mail would deliver it free of charge if postage has been prepaid, but would levy a fee, to be collected from the addressee, equal to double the amount of underpayment. Such postal revenue accrued to the country effecting delivery.
A necessary prerequisite was the determination of a universally accepted currency standard to enable member nations to equate the amount underpaid to their own currency. This standard was determined as the currency of the Latin Monetary Union – 100 gold centimes to the gold franc – and international rates were related to the gold centime accordingly.
The basic letter rate which was adopted was 25 centimes for the first 15g or half-ounce. From 1906 it was altered to 25 centimes per 20g or one ounce.
The failure to pay postage or the deficiency in the amount payable had to be notified by the transmitting to the receiving authority. This was done by applying a number of different handstamps with a “T” which stood for the French word “Taxe”, or by marking the item with a “T” in manuscript.