I did a little research on the EU website.
I assume no liability for the accuracy of this information.

In general, Airport Transit Visas are not needed for transit between two non-Schengen states.

There's a group of countries AFGHANISTAN, BANGLADESH, CONGO (Democratic Republic), ERITREA (4), ETHIOPIA, GHANA (5), IRAN (6), IRAQ, NIGERIA, PAKISTAN (7), SOMALIA, and SRI LANKA who require airport transit visas when transiting any Schengen country.

Then there is a group of countries that require Airport Transit Visas only in some countries.

Among these in India (from your other posts, I think your wife is an Indian citizen), citizens of which need an Airport Transit Visa when transiting Denmark, Germany, Greece, Spain, France, or Poland.

Pages 32-26 of the Common consular instructions on visas for the diplomatic missions and consular posts contains lots of info:
http://eur-lex.europa.eu/LexUriServ/...01:0149:EN:PDF

There were a few changes made to this effective May 1, 2008 that have not been updated in the above document:
http://eur-lex.europa.eu/LexUriServ/...046:01:EN:HTML