The company, which measures television audience, says the number represents just over half of the 37.8 million people who saw Obama's first inauguration in 2009. Eighteen networks aired some part of this year's inaugural ceremony on Monday.
According to Nielsen records, this year's ceremony was the second least-watched inauguration since 1969. It beat only President George W. Bush's second ceremony in 2005, which was seen by 15.5 million.
The most-watched inauguration since 1969 was President Ronald Reagan's first oath-taking in 1981, which was seen by 41.8 million people.
NBC had the largest number of viewers among broadcast networks on Monday, with CNN the cable news leader.
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