The picture below shows a Latvian postcard that is franked with a 6 Santimi definitive. It was sent from Riga to Liepaja (both large cities in Latvia). There is a Riga cancel 29.5.31 and a Liepaja cancel 30 V 31. The opposite side of the card (not shown) is a nice color landscape scene. What is most interesting about this postcard is the postage due marking.

The postage due marking is purple (PIEMAKSAT RIGA c) and has the number 18 written inside in red pencil. "PIEMAKSAT" means "to pay" in English. A better image of this marking is shown below.

Why was postage of 18 Santimi due?

According to Philatelia Baltica 90-92 1987 (by Birznieks/Zarins), the cost to send postcards from the period 1.02.25 to 20.12.31 was as follows:

The postcard was mailed with a 6 Santimi stamp. This should have been enough for postage anywhere within Latvia!

The first clue that we have is that Latvia has always charged double for postage due. Therefore, this postcard was missing 9 Santimi. In other words, the sender should have applied 15 Santimi worth of postage. Going back to Philatelica Baltica, we notice that the cost to send letters during this period was as follows:

Latvian Letter Rates (1.02.25 to 20.12.31)

local	6 Santimi
long distance	15 Santimi
foreign	25 Santimi

Our target rate of 15 Santimi is what would be paid for a letter between Riga and Leipaja because the long distance rate applies. If this postcard were a letter, then the postage due would make sense. But why would this postcard be charged the letter rate?

If we look closely at the message that is written on the left side of the card, we can see that it extends considerably into the address area on the right side. There were strict rules governing the use of postcards. If these rules were not followed, the postcard would be charged the higher letter rate. The sender of this postcard broke one of the rules. The message cannot extend into the address area of the card. Because it did, the "postcard" was considered a "letter" and was charged the higher rate.

There is at least one other rule governing the use of postcards that we know about. There is no writing permitted on the picture side of the postcard. If you know of any other postcard rules, or if you have similar postal history items, the author would be most interested in hearing about them.

Acknowledgements

Thanks to Masayuki Watanabe (Japan) for providing information about postage rates and double charges. Bill Apsit (Canada) provided the solution to this mystery when he suggested that there might be writing on the opposite side of the card causing it to be charged the letter rate. Thanks to Bernard Doherty (Australia) for providing general information about postage due regulations.