The BEL 20 is the benchmark index of Euronext Brussels and, as the name says, it holds about twenty companies. A reader can hold the whole list in their head, which is not true of the CAC 40, the DAX or the FTSE 100, and that legibility is genuinely useful. It is also the source of most of the mistakes people make when they use the index as a proxy for the country.
A narrow index is dominated by whatever is large inside it. A brewer, a bank and a small number of pharmaceutical and materials groups account for a large share of the total, and the index applies a cap so that no single name can grow without limit inside it. That cap tells you the problem exists. It does not make the index diversified.
The bigger issue is what the twenty names are not. Belgium's economy runs on medium-sized private companies, family holdings and cooperatives, and on the industrial and logistics activity around the ports. Very little of that is listed. The listed market skews towards a handful of internationally exposed groups, some of which earn the overwhelming majority of their revenue abroad. An investor buying the BEL 20 is buying global earnings that happen to be administered from Belgium.
That is not an argument against the index, which does the job it was built for. It is an argument against a specific inference. When the BEL 20 falls, the correct reading is usually that something moved in world beer, world pharmaceuticals or European banking. It is rarely that Belgian domestic demand has weakened, and the domestic economy can turn without the index registering it at all.
For anyone trying to read the country rather than the tape, the more informative series sit elsewhere: port throughput, the national bank's business confidence survey, and the wage index. None of them are quoted every fifteen seconds, which is precisely why they are worth more.