Saw this in a book last night. I hope I read it right and am remembering it right. If two rectangular coordinate systems share the same x axis and one is moving at a constant speed towards positive x and a beam of light is travelling along their x axes going towards the positive, then at the beam is at x=ct and x'=ct', one for each system. The book says then x-ct=M(x'-ct') for some constant M. What I don't get is doesn't x-ct=0=x'-ct' mean M is 1? What am I not understanding about this?