Directions are symbols added by Mystcraft. They are used to specify a horizontal direction, as might be described by compass directions. Most often these are used for the "rising" position of Suns, Moons, or Star Layers, but also for the position of a Rainbow. There are four symbols available: North, East, South, and West.
They combine much like Colors or Lengths; as a new Descriptive Book is read by the Grammar, a "current direction" is maintained, and each direction after the first combines compass-wise with the current direction. Thus "West, North" will indicate due northwest. Since the modifiers combine pairwise, "West, North, South" will be further south than "West, South, North". If it is begun with opposed directions, the results are undocumented, so if the direction starts with "North, South", it is not officially defined whether it will result in East or West for the result or next step.
Once another symbol has "accepted" the current direction, there will again be "no current direction" until a new direction is specified.