Let's say you're performing a campaign on Google AdWords and you have similar search terms -- for example, some with broad match and some with exact match for particular queries. Say, for example, that one of your searches is for [blue widgets] and the other is for blue widgets (broad match, no quotes). The question is -- which one does Google choose to trigger the ad?
A Google AdWords help document discusses this question in more depth. Depending on the criteria, different things may occur.
If there are multiple eligible keywords and one identical keyword, the common denominator keyword will trigger an ad.
On the other hand, if there are multiple eligible keywords in the same ad group (but no identical keyword), the keyword "that contains the most words" will trigger the ad.
Finally, if there are multiple eligible keywords across ad groups (but again, no identical keyword), the keyword with the highest combined Quality Score and CPC bid will trigger the ad.