Abraham Lincoln's "Gettysburg Address" and Robert E. Lee's "Letter to his Son" are both masterpieces of diction; that is, they both use language correctly and effectively.

One of the most effective phrases in the "Gettysburg Address" comes near the end:

But, in a larger sense, we can not dedicate,we can not consecrate, we can not hallow this ground.

Lincoln here announces the purpose of his speech: to dedicate and consecrate (make holy) the grave sites of the victims of the battle of Gettysburg.  By admitting that "we can not" truly accomplish this purpose, he magnifies the greatness of the victims' acts.  At the same time, he deflects attention away from himself; it is as if he is saying, "This is not about me, it's about the soldiers."

Lincoln draws attention to this sentence by using several simple, but powerful, poetic devices.  First, he uses anaphora, or repetition, of the phrase "we can not."  Second, he uses rhyme: dedicate and consecrate.  Third, he uses alliteration, the repetition of a vowel sound.  Listen to how many "hard c" sounds are in this sentence:

can (3x),dedicate, consecrate (2x in one word).

One of the most powerful sections of Lee's "Letter to his Son" is the following:

Duty, then, is the sublimest word in our language. Do your duty in all things....You cannot do more; you should never wish to do less.  Never let your mother or me wear one gray hair for any lack of duty on your part.

Lee drives home the importance of duty by one of the simplest yet most powerful methods: repetition.  In these 4 sentences, Lee uses the word "duty" 3 times.

Another method Lee uses here is assonance: the repetition of vowel sounds.  "Duty," which is used 3 times, is in assonance with "do," which is also used 3 times.

Lee's third sentence uses a device called antithesis:

You cannot do more; you should never wish to do less.

The sentence briefly contrasts the phrase "do more" with its opposite, "do less." 

Lee concludes his letter with an emotionally charged reference to the eventual old age of him and his wife, the boy's mother:

Never let your mother or me wear one gray hair for any lack of duty on your part.

Although some might see this as a guilt trip, it certainly adds power to Lee's instructions to his son.