Popular search terms tend to dominate the headlines, but there are literally millions of unique searches conducted every day, and savvy search marketers are taking advantage of the opportunities in the "search tail."

In fact, Google estimates that nearly 50 percent of all searches are one-of-a-kind. That's more than 100 million unique searches per day on Google alone.

These people aren't entering words like "travel" or "books." These are searchers who know what they want. These folks are searching for things like "virgin island bare boat charter company" or "Grohe plumbing supplies Aspen Colorado" It's hard to imagine all the variations people might enter into a search engine, and it's this incredible diversity that offers a significant opportunity for marketers.

You'll often hear this phenomenon described as the "search tail." If you're not sure what people are referring to, try visualizing a graph. Plot all search queries along the horizontal axis and plot the frequency of each query along the vertical axis. Place the most popular searches at the far left, followed by the somewhat popular searches, followed by less frequent searches, and finally the one-of-a-kind searches on the far right.

Here's what you'll find. There are only a few queries that have a very high frequency (yes, a lot of people search for Britney Spears, iPod and MP3). But the graph quickly flattens out into an extremely long "tail" that contains millions of less popular and unique searches. This tail is significant because cumulatively, it outnumbers the more popular searches at the head of the graph. In fact, if any marketer plots all searches conducted for a company, or a product, or a topic, a similar result will appear.

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