The A-minor motif involves the insertion of minor groove edges of adenines into the minor groove of neighboring helices. It has four subtypes depending on the position of the adenine to the interacting Watson-Crick base pair.
Type 0: The N3 of the A (or other) residue is outside the O2' of the far strand of the receptor helix. Type I: The O2' and N3 atoms of the A residue are inside the minor groove of the receptor helix. The inserted base for the Type I interaction must be an adenine. Type II: The O2' of the A residue is outside the near strand O2' of the helix and the N3 of the A residue is inside the minor groove. The inserted base for the Type II interaction must be an adenine. Type III: The O2' and N3 of the A (or other) residue are outside the near strand O2' of the receptor helix.
Reference: Nissen,P., J.A.Ippolito, N.Ban, P.B.Moore, and T.A.Steitz. 2001. RNA tertiary interactions in the large ribosomal subunit: The A-minor motif. PNAS USA 98:4899-4903.