I have noticed three ways to write the momentum 4 vectors i. P = (E/c, p) ii. P = (E, p) iii. P = (E, pc) The three ways to write the momentum 4 vectors is really confusing since I know how to derive the way it is written in the form as (i). But the two other forms (ii and iii) does not make any sense at all. Could someone please "enlighten" me in this matter?