For simplicity, this Demonstration considers just a single space dimension that, along with the time variable , gives a two-dimensional projection of Minkowski space. A stationary observer measures variables denoted and , while an observer in another inertial frame moving at a constant speed measures variables denoted ' and '. For ease of visualization, the primed variables are shown in red. These alternative variables are related by a Lorentz transformation, ' and '. Here is the rapidity, defined by . The velocity can also be expressed in dimensionless form as . In accordance with this hyperbolic transformation, the primed (red) coordinate axes are skewed at angles relative to the stationary axes. The origin represents an event in which momentarily for both observers. You can move the locator to determine another event, for which the two observers no longer agree on values of and . The two sets of values of and are represented by the black and red projections on their respective axes. Their numerical values are given at the bottom of the graphic. The constancy of the interval is shown by the relation . In the cases , , and , the interval is called timelike, spacelike, or lightlike (or null), respectively. Timelike intervals lie within the future or past lightcones, projected as yellow triangles in the graphic. The red lines meeting at the event point are parallel to their respective red axes. Note that time is not ordered in a spacelike event: past and future are not invariant; nor is space ordered in a timelike event: left and right are not invariant.