Pascall’s Triangle:

It is significant to note that coupling can only happen between non-equivalent protons of diverse chemical shifts. Coupling among the protons making up a methyl group is not possible as they are equivalent to each other. Likewise, coupling is not possible among the methyl groups of ethane because these groups cannot be distinguished from each other. It is also significant to remember that the coupling is transmitted via bonds and that the size of this coupling gets decreases with the number of bonds involved. Actually, coupling is rarely seen between protons that are separated from each other by much more than three bonds.

Figure: Pascall's Triangle

In effect, this means that most coupling occurs between non-equivalent protons on neighboring carbon atoms.