The law of wills and estates is fascinating and ridiculous and fascinating again:

(Deliberating on whether a signature on a separate piece of paper can be considered part of the will so as to render it legally valid.) If an unattached paper is to be admitted at all, there is much to be said in favour of an envelope, which may reasonably be held to have a far closer relationship to a document which it encloses than a second and wholly disconnected piece of paper. Envelopes are, by their nature, designed to have what may be described as a dependent and secondary existence, rather than an independent and primary life of their own.

Re Mann (England, 1942)