In recent decades, the U.S. Postal Service has insisted on enforcing its strong legal claim to ownership of the top right corner of all envelopes sent through the mail. That is where the postage goes. But in the early days after the invention of envelopes ("covers" to collectors), there were far fewer rules and regulations and little enforcement. Generally the postage stamp(s) were in fact placed in the upper right corner of the cover, and advertising "corner cards" or other devices were generally placed in the upper left corner. But there were many variations: all-over designs, ads in the lower left corners, "banner" printed material running across the entire top, advertising "collars" at the upper right with a space left for a stamp, etc. One of the scarcest type of early advertising cover has its advertising on the right side of the envelope, either at the top or the bottom.
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