When Did Stamps Require Placement In Ur Corner Of Envelope?

I can't find the reference at the moment, but in a recent thread on old covers from the late 1800's and early 1900's, a question was raised as to when there was an established protocol that stamps must be affixed to the upper right hand corner of an envelope.

I assume the requirement may have been implemented by different countries at different times and it occurs to me that there may have been a UPU regulation governing such things, when automatic cancelling machines came into being.

Anyway, I found this postcard on a web site (one of a number of similar postcards produced by various countries in that generation) that actually provided meanings as to what sentiment was being expressed depending on where the postage stamp was placed on the envelope:



Is anyone familiar with a specific date range when this sort of thing was considered unacceptable and the UR corner was specifically designated for postage?

My Grandfather (also a Stamp Collector) once told me when I was a kid that the placement of the stamp on a letter to Your Valentine had a specific meaning. Your post brought me back to a fun time in my childhood. Going through my latest finds with Grand-Pa and Dad. Now I get to make those same memories with my children. Thanks for the walk down memory lane!

This might be helpful.

From the Penny Black, at least.