If the difference between any term of a section and its predecessor is always equal , then that section is called parallel section . Example: 1 + 3 + 5 + 7 + 9 + 11 is a section. The first term of this section is 1, the second term is 3, the third term is 5, and so on. Here , second term - first term = 3 - 1 = 2, third term - second term = 5 - 3 = 2, fourth term - third term = 7 - 5 = 2, fifth term - fourth term = 9 - 7 = 2, Sixth term - fifth term = 11 - 9 = 2 So , the clause is a parallel clause.