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From Google:
Google ignores common words and characters such as "where" and "how", as well as certain single digits and single letters, because they tend to slow down your search without improving the results. Google will indicate if a common word has been excluded by displaying details on the results page below the search box.
I tried a few searches of phrases that we target for a site, and found that when I searched by leaving out the words that Google 'ignores' the resulting SERP is different (and the site in question does even better).
Too tired to pull anything more interesting from this, other than that it seems to dilute the above statement of its validity.
It starts with singular vs plural, but staring with msg 7 shifts to stop words.
e.g.
"Jobs London" is different to "jobs in London"
Google will give results for the second example for pages that have "jobs - space -a word - london" as opposed to "jobs london" where the keywords sit together.
Search the site search for more info.