TBAY: DRIBBLE SHADOWS (long)

[stickbook41]

No: HPFGUIDX 128836

HunterGreen mused:
Unless Dumbledore was lying or bending the truth there, he explained
Petunia's end of the deal in a letter, not in person. Also, he
implies (the words "charm I placed on you") that he personally placed
a charm on Harry, but that doesn't make sense either. 
<snippings>
My only theory (and I'll admit that its unlikely) is that he placed
the charm on the *letter*, and it became sort of like a contract. The
moment that she took Harry in (in other words made Privet Drive his
home, perhaps whenever her or Vernon cleaned out the cupboard and put
Harry there), that sealed the charm.

at which stickbook quips:
A few years ago there was a similar idea that was bandied about, but I
don't know that it ever got it's own proper acronym.  The theory was
that Petunia kept the letter and that ever since it's been hidden
under the first stair.  Or maybe it was the second stair, I can't
remember.  Anyway, that's why that stair creaks.  There was never any
real canon to go on, other than the fact that it seems to be mentioned
an awful lot for a single stair.  Does anyone else remember this
discussion?

It would be very interesting if that letter is the magical object that
insures the Dursley's house against WW unpleasantness.