Fused ring systems that share more than two atoms are called bicyclic molecules.

To name bicyclic alkanes, you follow these three steps:

1. Count the total number of carbons in the entire molecule. This is the parent name (eg. ten carbons in the system would be decane)
2. Count the number of carbons between the bridgeheads, then place in brackets in descending order. (eg. [2,2,1])
3. Place the word bicyclo at the beginning of the name.

Example: Name the following molecule by the IUPAC system of nomenclature.

1. Count the total number of carbons. In this case there are 7 carbons so the parent name is heptane
2. Count the total number of carbons between bridgeheads. In this case there are 2, 2, and 1 carbons between the bridgehead. Placed in descending order between brackets it is [2,2,1]
   
3. Put the word bicyclo in the front of the name
   

The name is thus bicyclo [2,2,1] heptane