91Ó°ÊÓ

The most common example of a fused ring system is decalin. There are two geometric isomers of decalin. The isomer in which the rings are fused using two cis bonds is the $cis\text{-decalin}$,and the isomer in which the rings are fused using two trans bonds is the $trans\text{-decalin}$.

While drawing, if the left ring is considered, the bonds to the right ring are directed downward, whereas the attached hydrogen atoms are directed upward. These bonds are cis, which can be called a cis ring fusion. One of the bonds to the right must be in an axial position, and the other is in an equatorial position.

The axial position is shown by a solid blue line, and the equatorial position is shown by a solid red line in the structures drawn.

$cis\text{-decalin}$