These waters are utilized as the temporary reservoirs of the superfluous polder water, each system of reservoirs being termed a boezem (bosom or basin), and all lands watering into the same boezem being considered as belonging to it.
The largest boezem is that of Friesland, which embraces nearly the whole province.
It sometimes happens that a polder is not in direct contact with the boezem to which it belongs, but first drains into an adjacent polder, from which the water is afterwards removed.
This is usually the case where there is a great difference in height between the surface of the boezem and the outer waters, and may be illustrated by the Alblasserwaard and the Rotte boezems in the provinces of South and North Holland respectively.
The height of the boezem peil ranges beween 13 ft.
The Schie or Delflands boezem of South Holland is of this kind, and such a boezem is termed besloten or " sequestered," in contradistinction to a " free " boezem.
A third kind of boezem is the reserve or berg-boezem, which in summer may be made dry and used for agriculture, while in winter it serves as a special reserve.