Why are external HDD enclosures advertised as 3.5"

[Andrew Jorgensen]

On 2/6/07, Michael L Torrie <torriem at chem.byu.edu> wrote:
> Anyway, this thread is pretty silly.  All we need to really know is that
> the 3.5" drives are standard desktop drives and 2.5" are laptop drives,
> actual dimensions and platter diameters notwithstanding.

I'm told, though I have no historical proof, that the 3.5" size was
based on the size of the front pocket on a dress shirt.  I've also
been told that 5.25" was based on the size of a common shipping
envelope.