Isomers have same chemical formula but different structures. For example, C₄H₁₀ possesses two isomers:

While only carbon skeleton shown, hydrogen atoms assumed present to make the 4 bonds required of each carbon.  Shown below are the 3 isomers for C₅H₁₂ and 5 isomers for C₆H₁₄:

Substitution of Cl for one H in C₄H₁₀ leads to 4 isomers with formula C₄H₉Cl:

End carbons and middle carbons are equivalent (in same environment) in C-C-C-C.
Note: Attaching Cl to equivalent carbons leads to the same structure!

The RIS (representing isomer structures) method uses arrows to reduce the number of skeleton forms.  The 4 isomers for C₄H₉Cl can be represented with 2 skeletons and 4 arrows:

The arrow shows Cl at 4 different positions to represent the 4 isomers attributed to C₄H₉Cl.The 8 isomers for C₅H₁₁Cl and the 17 isomers for C₆H₁₃Cl are represented below:

RIS can also be used when more than one Cl is present.  To represent the 9 isomers for C₄H₈Cl₂, fix one Cl with the skeleton and specify the other Cl with arrows:

 The 9 isomers for C₄H₈Cl₂increaseto 12 isomers for C₄H₈ClBr:

Now let's represent the 39 isomers for C₇H₁₅Cl; keeping track of all these isomers gets tedious but the task is made more manageable by using the RIS method.  After drawing the 9 skeleton forms of 7 carbons, position arrows to represent the 39 isomers:

The method can also represent enantiomers (mirror image twins). Here are the 11 unique structures for
C₅H₁₁Cl including the 3 sets of twins (T).