Periods can easily be moved from one slot in the timetable to another using drag & drop.
Empty green cells denote slots in the timetable where a lesson may be scheduled without risk of conflict. The different shades of green indicate the suitability of the positions for the dragged lesson. The darker, the better. The evaluation depends on the parameters you have entered (time requests, double periods, weighting, etc.).
Red fields indicate that the scheduling would be possible without clashes but the timetable would be significantly worse because of e.g. a blocking (time request -3). The details window shows which element is blocked. The timetable details window shows which element is blocked.

Cells highlighted purple indicate that the room allocated to the lesson is not available.
Move double periods and lesson blocks in the same way as you would move single periods. However, if you only want to move one period of a lesson block or a double period, press the <Ctrl> key prior to selecting the period you want to schedule. This means that the periods are then displayed as single periods and can be moved individually.
