State Archives of Belgium
These are Les archives de l’État en Belgique, or Het Rijksarchief in België.
Location: Rue de Ruysbroeck 2 Ruysbroeckstraat, Brussels
https://search.arch.be/fr/bibliotheque
Both in terms of physically archival storage, and in terms of access via the Internet, these establishments share functions, and databases. This pertains to Belgium's State Archives, its National Archives, and its Royal Library.
There is a related facility at Rue du Houblon-Hopstraat 26-28, the AGR 2 - Cuvelier, and it is these "Archives générales du Royaume", which are absolutely key to the finding of patent originals and patent catalogues which have not yet been digitized. (These nominal peculiarities lead to some challenges, "on the ground", as it were.)
As in the case of France, it is some of the very oldest material which, at least in catalogue terms, has been digitized, as per the link below. Finding minimal data here is fairly painstaking, but it is a great resource to have on hand.
