Think about the rhetorical devices in King's speech. Which device had the strongest effects?

Martin Luther King, Jr., used many rhetorical devices in his "I Have a Dream" speech, including tone, diction, figurative language, repetition, hyperbole, and allusions. To choose one device that made his speech most effective is difficult; each person would have to decide which one touches him or her the most. 

For me, one of the most moving parts of the speech is near the beginning where he combines an allusion to the Declaration of Independence with the metaphor of the "promissory note." This metaphor calls upon ethos (appeal to authority), logos (appeal to reason), and pathos (appeal to emotions) all at once. 

In a sense we have come to our nation’s Capital to cash a check. When the architects of our republic wrote the magnificent words of the Constitution and the Declaration of Independence, they were signing a promissory note to which every American was to fall heir.

This note was a promise that all men would be guaranteed the inalienable rights of life, liberty, and the pursuit of happiness.

It is obvious today that America has defaulted on this promissory note insofar as her citizens of color are concerned. Instead of honoring this sacred obligation, America has given the Negro a bad check, a check which has come back marked "insufficient funds."

But we refuse to believe that the bank of justice is bankrupt. We refuse to believe that there are insufficient funds in the great vaults of opportunity of this nation. So we have come to cash this check, a check that will give us upon demand the riches of freedom and the security of justice.

In this passage, by invoking the Declaration of Independence, King builds his credibility in that he is basing his whole argument on the founding document of our country. This adds great authority to his words. By describing the promises of the Declaration as a promissory note that has come back as an NSF check, he pictures the reality of the situation for blacks in America. This is an appeal to logic; it is obvious that the blacks are not being treated as equals, especially in the South. Finally, comparing what America has done to blacks to a "bad check" arouses our emotions. Writing a bad check is a crime; it deceives and steals from the person it was written to. It is unjust, and everyone knows that bad checks have to be made good. I find this section very moving.

The other rhetorical device that is similarly effective for me is King's use of repetition. In the "I have a dream" section as well as in the "let freedom ring" section, King punctuates his sentences by repeating these hopeful, inspiring words over and over again. The listener is tempted to join in and repeat the refrain with King. One gets the feeling of beating waves upon the shore--with each wave, the tide comes closer and closer to washing over the beach, just as we hope the time of equality for blacks will wash over all of America. In this section, King combines figurative language, a hopeful tone, and elevated, lyrical diction with the repetition, making an eloquent argument for freedom and opportunity for everyone.