According to the attachment of different substituent groups to the ring, PGs are named with capital letters such as A, B, E and F. In the same series, depending on number of double bonds on the side chains they are denoted by a subscript after the capital letter, e.g. PGE1, PGE2, PGE3, etc. Series 2 have two double bonds at 13-14 (trans) and 5-6 (cis). This is the most common variety.