Structural variants of double-stranded DNA: A form

The A form is a double helix, dextrorse (right-handed) like the B form, but wider. The fictitious line joining the phosphorus atoms defines the trajectory of the backbone; note its helicity.
Note also the inclination of the base pairs, stronger than in B-DNA.

Note that the base pairs are positioned on one side of the helix axis; this is the reason why the helix becomes wider.

Spacefilling model, made of (bases and backbone).
What was the major groove in B-DNA becomes in the A form a narrow and deep groove, while the minor groove is now wide and shallow.