If a tourist wants to visit europe with a schengen visa, the following rules apply:
- maximum contiguous stay is 90 days (in the following treated as 3 months)
- in any 180 day interval the sum of days must not exceed 90 days.
If somebody wants to visit europe as much as possible, the following visit pattern is possible: 3mi-3mo - 3mi-3mo... which means: 3 month in the schengen area, 3 months out and same again. This is xxx---
Also all these monthly patterns work for repeated visits (x=1 month in schengen area, - =1 month not in schengen):
xx-x--
xx--x-
xx---x
x-xx--
x--xx-
x-x-x-
Are all visit pattern allowed with 3 months in schengen and 3 months outside?
Now, what are the rules for combining patterns? For example it is obvious that a pattern like "xx---x" cannot be followed by "xxx---".