Employers can string fixed-term contracts together, but only up to a point set by law rather than by the contract itself. Workers on a second or third renewal often do not realise that conversion to permanent employment can happen automatically, or that a short gap between contracts does not reset the clock.
The chain rule (ketenregeling) caps it at three fixed-term contracts within three years — the fourth contract, or crossing the three-year mark, converts automatically to a permanent one, regardless of what the paperwork says. A gap between contracts of six months or less still counts toward that three-year clock; only a gap longer than six months resets the count to zero. There are exceptions for some young workers, BBL trainees and a handful of professions. Worth knowing: a 2028 change (not yet in force) will stretch that reset gap from six months to three years, specifically to close the loophole where employers "park" someone briefly and restart the clock — so the rule is getting stricter, not looser, if you're weighing whether to push for permanence now.