B-DNA is the most common type of DNA and exhibits a right-handed formation in its double helix.   Think of your right arm as the axis of the DNA with the backbone of the DNA wrapping around your arm from the shoulder to the fingers. Now, extend your arm out so its perpendicular to your body and keep your palm facing down.   Naturally, when you rotate your arm so that your palm is facing up, you will be rotating clockwise — this is the direction in which the double helix of B-DNA twists.   Repeating the same process with the left arm, the palm will naturally rotate counterclockwise.   DNA which has a left-handed formation is known as Z-DNA.   Z-DNA is not as common because it requires a high salt or aqeous hexammine environment to form.   However, there are some interesting nanomechanical applications involving switching between B-DNA and Z-DNA to move a mechanical arm based on the chemical environment.