In uk.d-i-y, marvelus wrote:
Slates have 3 holes, two about 2/3 way up
at the sides which you can nail some copper nails in and one at the
bottom in the middle. With that one a large rivet is inverted and slid
up between the two slates below and the wire part of the rivet enters
the hole from below and its bent down to secure the tile.
Look like this:
http://www.rylandswire.com/discrivets.htm
I dont know what they are talking about with "penultimate row", as
each tile has one.

This might be a regional variation. Round here the slates have just the
first two holes you mention.

Looking at the web page you quote it's clear that those rivets are
intended for top row slates only. Most slates are kept in place partly
by the weight of two tiles in the row above. However, for slates in the
top row, there is no row above. Hence the idea of clipping the top row
slates to the second-to-top row ("penultimate" row) with those rivets.

Of course it's possible that in some parts of the country *all* slates
are fixed that way, but I've never seen it.

--
Mike Barnes