Minecraft wiki entry for Argument_types, associated text: For the horizontal rotation (yaw), -180.0 for due north, -90.0 for due east, 0.0 for due south, 90.0 for due west, to 179.9 for just west of due north, before wrapping back around to -180.0.
For the vertical rotation (pitch), -90.0 for straight up to 90.0 for straight down.