Many of the world's 6000 to 7000 spoken languages are threatened with extinction, as Bernice Wuethrich discusses in her News Focus article “Learning the world's languages—before they vanish” (19 May, p. [1156][1]), and their disappearance will cripple attempts to probe the limits of linguistic diversity. But one crucial source of linguistic diversity is not mentioned in the article. Within the last 40 years, linguists have recognized that there are two major types of human languages: signed and spoken ([1][2], [2][3]). There are perhaps 200 to 300 signed languages, but there has been no thorough survey done. Linguists now understand that signed languages exhibit the basic properties that make a communication system a language. Research suggests that signed and spoken languages exhibit distinct patterns of variation ([3][4]). Although signed languages differ in their vocabularies, in word order, in the presence of auxiliary-like elements, and in other ways, they seem to be much less diverse typologically than are spoken languages. The relative uniformity of signed languages, in contrast to the typological diversity of spoken languages, may be due to the differing resources available to sign and speech, as well as to the differing perceptual and articulatory constraints imposed by the visual-gestural and oral-aural channels. This is a fundamental hypothesis about the factors that determine the available structures for individual human languages, yet it has hardly been tested. Doing so demands that we examine a large sample of signed languages. But like many spoken languages, the very existence of some signed languages is threatened. The pressures of educational policy, of more prestigious spoken and signed languages, and of the ease of communication across once-formidable barriers mean that many signed languages may disappear before we have the faintest understanding of how much signed languages can vary. For example, the indigenous signed languages of Southeast Asia are being replaced by signed languages substantially influenced by American Sign Language or French Sign Language ([4][5]). Understanding and preserving linguistic diversity will require that we investigate both major types of human languages, signed and spoken. 1. [↵][6]1. W. Stokoe, 2. D. Casterline, 3. C. Croneberg , A Dictionary of American Sign Language on Linguistic Principles (Gallaudet, Washington, DC, 1965). 2. [↵][7]1. U. Bellugi, 2. E. S. Klima , The Signs of Language (Harvard Univ. Press, Cambridge, MA, 1979). 3. [↵][8]1. K. Emmorey, 2. H. Lane 1. E. L. Newport, 2. T. Supalla , K. Emmorey, H. Lane, The Signs of Language Revisited (Lawrence Erlbaum Associates, Mahwah, NJ, 2000), p. 103. 4. [↵][9]J. Woodward, in ([3][4]), p. 23. [1]: /lookup/doi/10.1126/science.288.5469.1156 [2]: #ref-1 [3]: #ref-2 [4]: #ref-3 [5]: #ref-4 [6]: #xref-ref-1-1 View reference 1 in text [7]: #xref-ref-2-1 View reference 2 in text [8]: #xref-ref-3-1 View reference 3 in text [9]: #xref-ref-4-1 View reference 4 in text