I have been told that relative velocity can be negative on this forum, so i would like to ask a few questions based on this. scenario 1) Two observers O and O' are facing each other and are approaching each other. O sees O' approaching at v=0.5c and vice versa. The vector of the velocity is however pointing towards the back/negative x axis. So would the relative velocity be rather v=-0.5c in this case? Or does the sign of the relative velocity depend on if they are approaching or moving away from each other? Should this be the case, then is the relative velocity negative as they approach each other and then switches to being positive when they move away of each other? What about the case when they exactly meet. What would be the relative velocity in this case? scenario 2) Observer O is looking at observer's O' back while approaching. O' has his back turned towards O while he is approaching from behind. Is the sign of the relative velocity the same for both observers, or is the relative velocity different for each of them, as in having the sign switched?