The Gettysburg Address is as sacred an American text as "The Star-Spangled Banner." But as immediately recognizable as some of its words are, others are more elusive.

There exists no definitive edition of the speech Abraham Lincoln gave to dedicate a cemetery at the Civil War battlefield. Scholars still debate the speech's exact length, usually ranging from 268 to 272 words.

Tuesday marks the 150th anniversary of Lincoln's famous remarks. Like many of Lincoln's successor, President Barack Obama will not attend the anniversary gathering.

Lincoln historian Harold Holzer says the only explanation for why acclaimed orators like Obama and Bill Clinton would skip the anniversaries is the high bar Lincoln set.