Hello Johan and others,
For ages in parts of Europe (maybe in other places as well?) only few people had family names.
The people without it were differentiated from others with the same 'Given Name' by using a 'Patronymic'.
That's how you got your 'Dirk Jansz' which means Dirk, the son of Jan.
Up to now I have used the program Pro-Gen which handles patronymics very well.
Instead of the basic two fields for a persons name ('Given' and 'Family') it has three fields ('Given', 'Patronymic' and 'Family'). All with the same status.
It has allowed me and thousands of other users to work easily with many persons without family names.
(out of the 32.000 people in my database, about 20.000 have no family name).
GRAMPS knows a name item 'Patronymic' but apart from mentioning it in the window top bar and using it in other places, unfortunately it is somewhat hidden in a pop-up name editor window.
It would be much better if in GRAMPS the 'Patronymic' would get the same status as 'Given' and 'Family' and show up as one of the entries in the 'Prefered name' fields. Maybe as on option? 
It would certainly make life a lot easier for the people who have many persons with only patronymics in their database.
So I suggest to 'promote' the 'Patronymic' entry to the 'Prefered Name' section in the Person-Window. Would that be possible?
Fred Reenders