page 91, line 11 from top

http://books.google.ru/books?id=9pGGZsRHXlwC&lpg=PA45&dq=anatomy of a wall shelf&hl=ru&pg=PA47#v=onepage&q=middle shelf&f=true

одназначно medium - это брак

ЗЫ. более того, даже если бы был вариант middle, он бы мог быть в некоторых случаях неверным (если средних полок больше одной, то нужен неопределенный артикль "a")

If there are (say) seven shelves, it's:
* top shelf
* 2nd shelf from the top (or 2nd shelf down)
* 3rd shelf from the top (or 3rd shelf down)
* middle shelf
* 3rd shelf from the bottom (or 3rd shelf up)
* 2nd shelf from the bottom (or 2nd shelf up)
* bottom shelf.

If there's an even number of shelves (4/6/etc), the same names apply -- although there won't be a middle shelf, of course.

(I do like 'mediocre shelf' and 'so-so shelf' though -- hilarious!)

really?
always?
how about here:

Similarly, if you had three separate houses in a straight line at the side of a road -- Nos 1, 2 and 3 -- which were different distances apart, you'd still refer to house No 2 as the 'middle house'.

'Middle' (adjective) and 'the middle' (noun) are usually only approximate. If you wanted to say that house No 2 was EXACTLY in the middle between houses 1 and 3, you'd have to use a more specific term such as:

'House No 2 is ....
* exactly half-way between houses 1 and 3'
or, to use a much more technical term,
* equidistant from houses 1 and 3'.

as the number increases though, it's not so certain to me. i bet many people, when asked to locate the "middle item" among many others, would just pick the one closest to the geometrical center of the group, i.e. they would end up with an item equidistant from the extreme ends -- simply because this would be much quicker and easier than to count all the items meticulously.....

no point in arguing over this further, of course; clearly this is a matter deserving its own full-fledged scientific research -- grants, publications, and everything in between.