Sources: Marriage supplements
If two people wanted to marry, they had (and still have) to supply a few documents. These documents, the huwelijksbijlagen (marriage supplements) were kept with the marriage acts of the civil register.
Why were they made?
Before a wedding, the registrar had to verify the couple was indeed allowed to marry. They had to be of a certain age (in the 19th century that was 18 for the groom, 16 for the bride), they should not be married already, if they were under 30 they needed permission from their parents, men must have fulfilled their military obligation. A couple that wanted to marry had to supply the necessary documentary evidence.
What information do they have?
At least birth certificates of the bride and groom (or baptismal certificates, if they were born before 1811). Usually, there is also a document stating the groom has fulfilled his military obligations (which either meant he has completed his military service, or he was not drafted).
If you're lucky, there can be many other documents. If the parents were dead (and the spouses were under 30) you will find the death certificates of the parents (and maybe even of the grandparents, as they had to give permission if the parents were deceased). If the parents could not be present, they could give their permission in an act made up by a notary, this act will be included with the marriage supplements. If the parents refused to give their permission, the couple could apply for a court decision, allowing them to marry without parental permission, and an act drawn up by the court will be part of the marriage supplements. If one of the spouses married before, they had to supply a death act of their previous spouse (or a divorce act).
Other documents are rare. Marriages between certain relatives were only allowed with dispensation from the king, you may find a royal decree granting this permission (this was needed, for example, when Stephanus Johannes Pardoen married his aunt Theodora Margaretha Wilhelmina Pardoen). If no birth or baptism act could be obtained, there may be an acte van bekendheid (act of acquaintance), drawn up by the court and signed by four witnesses, stating that the subject of the act is who he claims to be.
Where can I find them?
Marriage supplements were kept by the district courts, and are now at provincial archives (for the province Zuid-Holland they are kept in the Nationaal Archief in The Hague). You can find the addresses of the provincial archives on the regional pages on the Trace your Dutch roots website. Only in a few cases will you find them elsewhere, e.g. the marriage supplements from Rotterdam for the period 1812-1852 are on the website of the Rotterdam city archive.
In a few cases, the marriage supplements are lost. The archives of the courts of Hoorn and Alkmaar, for example, were lost by fire in 1890. Losses include the marriage supplements of the region. Marriage supplements of the The Hague and Leiden region were lost in March 1945, when the British air force accidentally bombed the Bezuidenhout region in The Hague.