'Mercury'

"I was half-way through the belt of trees above the water-meadow when automatically my hand went to my pocket, encountering the sharp edge of the flap of the unsealed envelope.  With no further intention in my mind I pulled it out and looked at it.  There was no address (or direction, as Mrs Maudsley called it, why I could not imagine) on the envelope: there never was.  But the open flap disclosed some writing which, at the moment, was the wrong side up.
 Among the complexities of our school code was a very wholesome respect for the Eleventh Commandment.   But we also had a strong sense of justice, and if we were found out we did not expect to be let off.
…
The rules about reading other people's letters were fairly well defined.  If you left your letters lying about and somebody read them, then it was your fault, and you were not justified in retaliation,  If somebody rifled your desk or locker and read them then it was their fault, and you were justified in taking vengeance…
...I had often passed round notes at school.  If they were sealed I should not have dreamed of reading them; if they were open I often read them -- indeed, it was usually the intention of the sender that one should, for they were meant to raise a laugh.  Unsealed, one could read them, sealed one couldn't: it was as simple as that.  The same rule applied to post-cards: one read a post-card that was sent to someone else, but not a letter.
Marian's letter was unsealed and therefore I could read it.  So why hesitate?
….
But I would not take the letter out of the envelope: I would only read the words that were exposed, and three of them were the same, as I could see from upside down.

   'Darling, darling, darling,
        Same place, same time, this evening,
    But take care not to---'

The rest was hidden by the envelope."

The Go-Between  L.P.Hartley