Menzerath's law, or Menzerath–Altmann law (named after Paul Menzerath and Gabriel Altmann), is a linguistic law according to which the increase of a linguistic construct results in a decrease of its constituents, and vice versa  .
E.g., the longer a sentence (measured in terms of the number of clauses) the shorter the clauses (measured in terms of the number of words), or: the longer a word (in syllables or morphs) the shorter the syllables or words in sounds.
According to Altmann (1980), it can be mathematically stated as:
- is the constituent size (e.g. syllable length)
- size of the linguistic construct that is being inspected (e.g. number of syllables per word)
- , , are the parameters
The law can be explained by the assumption that linguistic segments contain information about its structure (besides the information that needs to be communicated).  The assumption that the length of the structure information is independent of the length of the other content of the segment yields the alternative formula that was also successfully empirically tested. 
Beyond quantitative linguistics, Menzerath's law can be discussed in any multi-level complex systems. Given three levels, is the number of middle-level units contained in a high-level unit, is the averaged number of low-level units contained in middle-level units, Menzerath's law claims a negative correlation between and . Menzerath's law is shown to be true for both the base-exon-gene levels in the human genome , and base-chromosome-genome levels in genomes from a collection of species .
- Gabriel Altmann, Michael H. Schwibbe (1989). Das Menzerathsche Gesetz in informationsverarbeitenden Systemen. Hildesheim/Zürich/New York: Olms. ISBN 3-487-09144-5.
- Luděk Hřebíček (1995). Text Levels. Language Constructs, Constituents and the Menzerath-Altmann Law. Wissenschaftlicher Verlag Trier. ISBN 3-88476-179-X.
- Gabriel Altmann (1980). "Prolegomena to Menzerath's law". Glottometrika 2: pp. 1–10.
- Reinhard Köhler (1984). "Zur Interpretation des Menzerathschen Gesetzes". Glottometrika 6: 177–183.
- Jiří Milička (2014). "Menzerath's Law: The whole is greater than the sum of its parts". Journal of Quantitative Linguistics 21 (2): 85–99. doi:10.1080/09296174.2014.882187.
- Wentian Li (2012). "Menzerath's law at the gene-exon level in the human genome". Complexity 17 (4): 49–53. doi:10.1002/cplx.20398.
- Ramon Ferrer-I-Cancho, Núria Forns (2009). "The self-organization of genomes". Complexity 15 (5): 34–36. doi:10.1002/cplx.20296.