The quote from the three witches, "Fair is foul and foul is fair," echoes throughout the story, and is the backbone of the many 'switches' that occur between and amongst the characters and their positions. Macbeth's opening line reinforces this theme with, "So foul and fair a day I have not yet seen." He describes the day as foul after having to brutally slay so many men. The day is fair because of his absolute triumph and assured rewards. This, as with many things in the play, see-saws back and forth: his fair winnings and heightened position turn foul again by the end of the play.
Possibly the most notable switch occurs between Macbeth and Lady Macbeth. When Lady Macbeth learns of the witches' prophecy, she is absolute in her decision to kill the King. Macbeth, while he clearly likes the idea, and even shares her desire, falters on holding his promise to her until she threatens his manhood directly. After he kills the King and Banquo (separately) he is distraught with shame and guilt, while Lady Macbeth holds herself together and covers for his strange behavior. In Act V, we see Lady Macbeth falling apart, a downfall we later learn leads her to suicide. Macbeth, on the other hand, has forgotten his guilt, and is even willing to fight in the face of certain death when he learns of Macduff's unmotherly birth. While both characters may be viewed as foul, the theme still applies. One would expect, stereotypically, that Macbeth would be the one trying to convince his queasy wife that killing the King would be a blessing. Instead, Shakespeare turns things upside down and puts the pants on Lady Macbeth. Just as we're beginning to accept this, he turns it around again, with Lady Macbeth's suicide and Macbeth's heroic (although evil) bravery.
Act IV contains two noticeable echoes of the "Fair is foul and foul is fair" theme. First, while Malcolm and Macduff are talking, we learn of Malcolm
's terrible nature,...