John Donne uses an unusual metaphor to describe being separated from his beloved in "A Valediction: Forbidding Mourning." He likens the souls of himself and his lover to the two legs or feet of a compass. A compass is a drawing tool that allows people to draw perfect circles. It consists of two legs, or thin metal poles. One is placed in the center of the circle to be drawn, and the other, with a pencil attached, is used to draw the circle.
The two legs/feet, or poles, are attached at the top. Donne is saying that since he and his beloved are like the two legs of a compass, no matter how far apart they are from one another, they will never truly be separated. They will always move in harmony. In the stanza below, he compares his beloved to the foot of the compass that stands still in the center of the circle and himself to the foot that draws the circle:
If they [the souls of the lovers] be two, they are two [souls] so
As stiff twin compasses are two;
Thy [your] soul, the fixed foot, makes no show
To move, but doth, if the other do.
This type of unusual metaphor, which causes a reader to think, is a major trait of metaphysical poets like Donne.