This isn't homework, nor is it an exercise problem; merely a question about a diagram. Re: B.Schutz book "A First Course in General Relativity" 2nd Edition, (Asian print version), page 5, Figure 1.1 "A spacetime diagram in natural units". From section 1.4 Spacetime diagrams: A world line is the locus of events observed by the reference frame as a particle moves with velocity v. The vertical axis is t. The horizontal axis is x. The equation is: slope=dt/dx = 1/v where v=1 is the speed of light. When v=1, the slope of the world line is positive at 45 degrees. When v>1, the slope is positive but less than 45 degrees. Q: The World line for |v|<1 has a negative slope - goes from upper left to lower right. Can someone explain why this slope is negative and why not positive greater than 45 degrees? Lou.