The Flemish Parliament has 6 constituencies, the 5 Flemish provinces and Brussels. The Brussels Parliament is a single constituency parliament as is the parliament of the German-speaking community. Members of the Walloon Paliament are still elected in the old arrondissement-based constituencies.
An electoral threshold is the minimum share of the a party list requires before its candidate(s) can be elected to office. Here in Belgium this is 5%.
The effect of an electoral threshold is to deny representation to small parties or to force them into coalitions or cartels. The idea behind the threshold is to make the political system more stable and reduce the number of fringe parties. However, critics of electoral thresholds says that they can serve to disenfranchise supporters of minor parties.
In addition to the 5% threshold that is has been in force in parliamentary elections for the past 16 years, the number of seat available in a given constituency creates a natural threshold. In constituencies with less than 20 seats the natural threshold will be higher than 5%.
For example in a constituency with 4 seats a party would normally need to get more than 20% of the vote in order to get a representative elected. In some exceptional circumstances the natural threshold could be a little lower. In addition to the number of seats available, the seat allocation formula used (in the case of parliamentary elections in Belgium D’Hondt) and the number of parties standing can also influence where the natural threshold is set.
However, as a rule of thumb, smaller constituencies have higher natural thresholds and larger constituencies have lower ones
Furthermore, if more votes are cast for parties that do not win any seat, that will mean a lower proportion of votes will be needed to win a seat.
The effect of the 5% threshold in practice
For the Flemish Parliament, the 5% threshold has an impact in 4 of the 6 constituencies. However, in Limburg that elects 16 MPs to the Flemish Parliament it is the natural threshold that is decisive in whether a party gets an MP elected. In the Brussels constituency where 6 members of the Flemish Parliament are elected the natural threshold is (depending on circumstances) around 16%.
In the elections to the Federal Parliament, where fewer MPs per constituency are elected than is the case for the Flemish Parliament, the 5% electoral threshold only comes into play in two Flemish constituencies: Antwerp and East Flanders. In West Flanders, Limburg, Flemish Brabant and Brussels, the natural threshold is higher than 5%.
This is also the case with regard to the elections for the European Parliament where the natural threshold is around 8.33% for lists standing in the Dutch Language Constituency and 12.5% for lists standing in the Francophone Constituency. As there is only one German-Speaking Belgian MEP, he or she effectively elected on a first past the post system.