Diction is simply the choice of words and phrases a writer or speaker uses to make his point; in this case, both Abraham Lincoln and Robert E. Lee use diction which is perfectly appropriate to the occasion and the audience of their works.
"The Gettysburg Address" is a short but stirring speech which Lincoln gave during the Civil War. Lincoln uses language to remind his listeners that America is still one country. "All men are created equal" comes from the Constitution, and his reminder that America has a "government of the people, by the people, for the people" echoes many Constitutional principles. Lincoln is serious and heartbroken at the losses he is at Gettysburg to commemorate, but he is also determined that the men lying here will not have "died in vain." He uses words like "freedom" and "liberty" to stir up patriotism, and he uses sacred language like "consecrated," "holy," and "hallow" to honor the loss of life. He uses persuasion when he reminds us that we must all dedicate ourselves to ensuring that these deaths were not "in vain." Lincoln displays a masterful use of language to set the proper tone of sorrow, responsibility, and hopefulness for the future.
Though he and Abraham Lincoln are on opposite sides of the Civil War, Robert E. Lee also uses effective and appropriate diction in "A Letter to His Son." Like Lincoln, Lee writes succinctly and plainly, so there can be no mistake, about how he wants his son to conduct himself so neither of his parents "will wear one gray hair for any lack of duty on [his] part."
The letter is written in short paragraphs which seem almost like biblical proverbs, words by which his son should live. He says, for example:
Never do a wrong thing to make a friend or keep one; the man who requires you to do so is dearly purchased at the sacrifice.
Lee's advice is simple and useful, designed to foster good character in a young man. The language is persuasive, like Lincoln's, and Lee is clear that his son say, do, and live according to a few important principles which he clearly delineates.
Both works end with a call to duty, one to a divided nation and the other to a devoted son; and both men had a clear understanding of the language that would be most effective for their purposes and their audiences.