In Belgium married couples have, by default, community property (gemeenschap van goederen / communauté de biens) with the exception of  everything they owned before the marriage. Usually what one spouse receives or inherits from his family remains personal property and does not become community property.

Community property means that they own everything they acquire during their marriage in equal shares.  When one of them dies, their half of the community is inherited by their heirs, usually the children. If there are no children, the surviving spouse would inherit their half of the community property. 

In their marriage contract, a couple can agree that the surviving spouse will receive more than their half of the community property. If so inheritance tax will be due on what they receive in excess of their half.