Quote:
Is there a period of time these occurred? It seems to me that this practice would make post office book keeping tough as the half stamps might not be accounted for.
Bisects have been used since the very first US stamp issue. There were general guidelines/rules, but I don't want to go into the postal history. But basically, bisects were used when the post office ran out of the lower denomination stamp. Therefore, the first US general issue bisect was with the 10c Washington (US#2), to take the place of the 5c Franklin (US#1).
Bisects are not always diagonal. In fact, US#2 exists as both horizontal and vertical bisects on cover, in additional to diagonal. Sorry, I do not have scans of those covers. They are quite valuable $10K+ and are very rare.
In general, bisects are NOT common, and even the basic ones on full cover can command $100+. Bisects cut from cover have much lower value, because they are easier to forge.
My memory is fading, but the last issue that I recall seeing existing as a bisect on cover was either from the Presidential Series or the Liberty Series.