Dutch family names were not required until 1811 when emperor Napoleon annexed the Netherlands;[1] prior to 1811, the use of patronymics was much more common.
Although these words, tussenvoegsels, are not strictly essential to state the person's surname, they are nevertheless a part of the surname and are almost always included for clarity.
For example, someone whose family name is "De Vries" is not found at the letter "D" in the telephone directory but at "V"; the "de" is a tussenvoegsel and is not a part of the indexing process but rather is more of a stylistic qualifier.
Another reason for this methodology is that it makes finding someone's name in a database relatively easy, since most Dutch prepositions start with the same letter (and thus if the prepositions led, there would be constant superfluous data entry to arrive at the desired name).
In the Netherlands, the tussenvoegsel is written with a capital letter if no name precedes it.