Lock a period in place by clicking on <Lock period> in the tool bar of the timetable. This will prevent the automated scheduling tool from moving the period during a subsequent optimisation run. Alternatively, lock the selected period by pressing <F7>. The same key can be used to unlock a period.
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Locked periods are marked with an asterisk (*) in the period details window next to the lesson number. You can also choose to display the asterisk in the timetable period (on the "Layout 2" tab under <Timetable Settings>).

Tip:
You can see all locked periods in the window "Lessons | Locked Lessons". This window also allows you to unlock the lessons at the different levels. Please refer to chapter "Locked lessons" for more details.