Fair is foul and foul is fair

To remember this quote use the following mnemonic.

A fair lady (blond)

Is fouled

That's a foul says the referee

Sent off is only fair

"Fair is foul and foul is fair," is a particularly well known Shakespeare quote said by the three witches at the end of act 1 scene 1.

It tells us what to expect from the play. The inversion of values - good becomes evil and evil becomes good. This theme runs right throughout the play. It means appearances can be deceptive, it's a paradox. 

The fog and filthy air the witches create replaces the freshness and lightness of Scottish air. 

The witches are symbolic of foul but give fair advice (but always with consequences).

Macbeth transformed from good man into pure evil. 

Note: The very strong use of alliteration using the letter F: Fair is foul and foul is fair.