If you select a period in the timetable and move it by holding down the left mouse button, already scheduled periods may show certain features:
a) No marking of the period
If you want to drag&drop Tu-5 of class 1a to Tuesday first period, you see that the first period is not marked, i.e. it is not suitable for a swap, because it would collide with the period. If you still drop it on Tu-1, Untis will ask you if you want to save the block and un-schedule the clashing lesson or if you want to create a clash.
b) blue arrow
If a scheduled period has a blue arrow in the left top corner, a swap chain is possible with this period. In our example this would be e.g. Wendesday first period. In the timetable the swap chain is symbolised with red arrows.
If you drop the period and confirm, the periods will be swapped.
c) blue arrow and green background
Periods with a green background are also suitable for 2 period swaps (e.g. Th-1). If you drop a period on such a position, in the appearing dialogue you can decide if you want to have the 2 period swaps (Tu-5 with Th-1) or if you want to make consecutive swaps.