The Rev. Dr. Martin Luther King Jr.’s speech at the March on Washington for Jobs and Freedom in 1963 was unusual among great American speeches in that its most famous words — “I have a dream” — were improvised. -  Drew Henson, NY Times

Without question, Dr. King's "I Have A Dream Speech" is not only one of the most famous speeches of American history, it is one of the most iconic moments. And it almost didn't happen.

Several historians and friends of Dr. Martin Luther King Jr. have shared that at the most critical and crucial moment of Dr. King's Speech, he went off script, stumbled for just a moment, then, with the encouragement of Mahalia Jackson, shared his beloved dream.

"King read from his prepared text for most of his speech," Henson writes, relying heavily on "the Bible, the constitution and the Declaration of Independence - just as President John F. Kennedy had a few months earlier."

But according to Economist Tim Hartford, Dr. King never seemed satisfied with what he had. In addition to staying up late the night before, editing and re-editing, he also scratched and marked his speech in the back seat of the car on the way to the Washington Memorial and even on stage while waiting his turn. But even then, Dr. King knew something was missing. So about six minutes into his speech, Dr. King looked down at the script, his well crafted but "a little bit lifeless" script and realized it wasn't working.

So he improvised.

The line Dr. King was supposed to say was "Go back to our communities as members of the international association for the advancement of creative dissatisfaction." 

Instead, he says:

Then, Dr King paused.  The people behind him knew he no longer was on script, and it was then that history was made. 

Mahalia Jackson, the Queen of Gospel, had long been a supporter of Dr. King and the "no-famous bus boycott that lunged the modern Civil Rights Movement," and she had heard him, on more than one occasion, tell his dream of "seeing little Negro boys and girls walking to school with little white boys and girls, playing in the parks together and swimming together"(History.com). And she knew the people needed to hear it.

When Dr. King begin to speak from the heart and not the script, when she sensed a brief pause of thought, she yelled out, "Tell 'em about the dream Martin."

So he did.

You can read his scripted/unscripted script here.

"When we allow freedom to ring-when we let it ring from every city and every hamlet, from every state and every city, we will be able to speed up that day when all (If God's children, black men and white men, Jews and Gentiles, Protestants and Catholics, will be able to join hands and sing in the words of the old Negro spiritual, "Free at last, Free at last, Great God a-mighty, We are free at last."