Possible spin orientations due to three protons:

In the same way if we focus on the methylene protons and look for the possible spin combinations of the methyl group protons we find that there are eight such combinations. As you can notice, these combinations fall in four groups. Accordingly, the methylene protons signal splits into four lines (quartet) having an intensity ratio of 1:3:3:1

Figure: Possible spin orientations due to three protons of methyl group

Same, we can take other examples of molecules in that a given category of proton has one, two, three or more equivalent neighboring protons. It has been found in which the number of lines in a signal (multiplicity) is governed by (n+1) rule. A given signal would split into n+1 lines where n is the number of equivalent protons on the adjacent C atom(s).