Just now I was opening envelopes containing 'classified ad' forms and mused how rare it is to damage the contents of an envelope with a paper knife.
This caused me to pay more attention to the process and I noticed that the the way the envelope 'leaf' folds and overlaps the bottom of the envelope covers the contents, guiding the letter knife so it cannot easily get 'inside' the contents and damage them. Envelopes with long thin leaf (instead of the traditional shape) seem to have a longer tab at the ends to fulfill the same function, though the letter knife goes in one layer 'deeper'.
It made me realise that tapping an envelope to move undersize contents down to the end is a good way to minimise the risk of tearing the contents.
Is this by accident or design?
<Yes, typing out ads is one of the most boring bits of editing MEW!>
Neil