Proportional representation system in Belgium was adopted in 1899 and has not been changed since then. The reforms to the electoral system over the years were about the issues such as changing the size of the assembly, distribution of seats among electoral districts, 5% threshold at the district level (2003) and others.
Belgium, Constitution (1831, last amended in 2014), accessed 27 May 2019
ARTICLE 62. The composition of the electoral colleges is regulated by the law. Elections take place in accordance with the system of proportional representation that the law determines. Voting is obligatory and secret. It takes place in the municipality, except in the cases determined by the law.
ARTICLE 63(1) The House of Representatives is composed of one hundred and fifty members. […]
For further details of the electoral system in Belgium refer to:
Belgium, Election Code (12 April 1894, last amended in 3 November 2017), [in French] accessed 27 May 2019