Comparison between a spectrum S={2, 9, 11, 13} with a PTM and a spectrum T={2, 6, 8, 10} without PTMs. Spectrum S has a PTM of 3 when compared with spectrum T. The PTM is represented as three consecutive 0's in the shaded area in the vector representation of S. The second and third 1's in S are shifted by 3 to the right when compared with T. (a) There is only one matched mass pair between S and T (the mass pair is connected by a dotted line). (b) There are two matched mass pairs between S and T(δ=3)