Questions and answers round about letter-openers

What's the right material for a letter-opener?

There are letter-openers in metal, wood, horn, ivory, bone, glass or any other material you like.
They all are good to use. The crucial property is a sufficient hardness to tear up the paper in the crease of the envelope.

Has a letter-opener to be sharp?

No, it must not be really sharp because it has to tear up the paper, not to cut.
This has the following reason: the letter-opener is leaded only by the crease. A sharpened edge will cut the wings of the crease as well as its bend. By tearing up the crease the resinstance of the bend is much lower than in the unfolded part of the paper. So the tearing tool takes the way of lowest resistance.

May I open a parcel with a letter-opener

Please take a pair of scissors, a knife or a cutter to do so.

What's the right size for a letter-opener?

There is no rule.
You get the same success in opening a letter with my small opener "Mini" as with a bigger one like "Gerd".
Of course: big hands need big tools.
But more decisive than this will be the aesthetic aspect.