Dual franking is usually the result of

a) paying for a special service in the destination country. Usually Special Delivery or airmail (in the early days). Major post offices often kept the foreign stamps on hand for these special services.




b) the more frequently seen dual franking is a redirection. There are rules about whether redirection is a free service or not, that I am not up on. One I am sure of is that if the letter is opened and redirected it required new postage. However, in any case, many recipients would not have been aware that redirection service was free (in some situations) and paid the postage anyway.





c) then, of course, there are the philatelic covers, such as 2-country first flights, joint issue FDCs etc





d) and lastly (that I can think of) are the situations where stamps of one territory, colony or country became useable in another after amalgamation (confederation, call it what you will). Germany and DDR are one example, Newfoundland and Canada are another, after April 1, 1949





Roy

 
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Excellent post, Roy!

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