An intermediate switch is used to provide an extra level of control over and above two-way switching (two switches controlling one light - usually in a hall/landing set up with one switch upstairs and one downstairs).
An intermediate switch sits between the two "end" switches and gives an extra level of control for Three Way Switching - Two intermediate switches between the end switches will give 4 way switching & there is no limit - if you wanted you could have 100 way switching with 98 intermediate switches.
You can not arrange two way switching with just one regular switch and one intermediate switch.
An intermediate switch is just a changeover switch - it has four terminals. A1 will connect to A2 and B1 will connect to B2 - When the switch is thrown A1 Connects to B2 and B1 to A2 Like this :=: & :x:
There are some diagrams here...
http://en.wikipedia.org/wiki/Multiway_switching
The colours Red/Black do NOT indicate Live/Neutral
Some of the terminology is Very American - particularly the expressions 3 way & 4 way.
Peccavi
August 2010