The lake muds consist of three main components: 1) organic matter (humus), 2) minerogenic matter, 3) an inorganic, biogenic component (diatom frustules and calcium carbonate). For the investigation of such sediments chemical analysis is necessary, since common petrographic methods are not sufficient. The organic matter is best expressed as loss on ignition, the inorganic biogenic components as alkali-soluble SiO 2 and calcium carbonate, and the minerogenic component as the difference between total SiO 2 and alkali-soluble SiO 2 . A distinction between two types of humus is of greatest importance. This can be expressed as the content of organic C and by the C/N ratio. If the content of organic C is less than 50% and the C/N ratio less than 10, the humus is neutral, and if the content of organic C is higher than 50% and the ratio C/N is higher than 10, the humus is acid. Triangular diagrams indicate the relations between lake types and sediments, and are also used to illustrate the development of two Danish lakes.