An isogloss—also called a heterogloss (see Etymology below)—is the geographical boundary of a certain linguistic feature, such as the pronunciation of a vowel, the meaning of a word, or use of some syntactic feature. Major dialects are typically demarcated by groups of isoglosses; for example the Benrath line distinguishes High German from the other West Germanic languages; and the La Spezia–Rimini Line divides the Northern Italian dialects from Central Italian dialects. However, an individual isogloss may or may not have any coincidence with a language border. For example, the front-rounding of /y/ cuts across France and Germany, while the /y/ is absent from Italian and Spanish words that are cognate with the /y/-containing French words.
One of the most well-known isoglosses is the centum-satem isogloss.
Similar to an isogloss, an isograph is a distinguishing feature of a writing system. Both concepts are also used in historical linguistics.
Centum-Satem isogloss 
The Centum-Satem isogloss of the Indo-European language family relates to the different evolution of the dorsal consonants of Proto-Indo-European (PIE). In the standard reconstruction, three series of dorsals are recognised:
In some branches (for example Greek, Italic and Germanic), the palatals fell together with the velars: PIE *keup- "tremble (inwardly)" became Latin cupiō "desire" and *ḱm̥tom "hundred" became Latin centum (pronounced [kentum]); but *kʷo- "interrogative pronoun" became quō "how? where?". These branches are known as Centum branches, named after the Latin word for hundred.
In other branches (for example Balto-Slavic and Indo-Iranian), the labiovelars fell together with the velars: PIE *keup- became Vedic Sanskrit kopáyati "shaken" and *kʷo- became Avestan kō "who?"; but *ḱm̥tom became Avestan satəm. These branches are known as Satem branches, named after the Avestan word for hundred.
North-Midland isogloss (American English) 
A major isogloss in American English has been identified as the North-Midland isogloss, which demarcates numerous linguistic features, including the Northern Cities vowel shift: regions north of the line (including Western New York; Cleveland, Ohio; lower Michigan; northern Illinois; and eastern Wisconsin) have the shift, while regions south of the line (including Pennsylvania, central and southern Ohio, and most of Indiana) do not.
Northwest Semitic 
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A feature of the ancient Northwest Semitic languages is the following: w- > y- (w becomes y at the beginning of a word). Thus, in Proto-Semitic and subsequent non-Northwest Semitic languages and dialects, the root letters for a word for "child" were w-l-d. However, in the ancient Northwest Semitic languages, the word was y-l-d, that is, with w- > y-.
Similarly, Proto-Semitic ā (long a) becomes ō (long o) in the Canaanite dialects of Northwest Semitic. Note that within the Aramaic languages and dialects of Northwest Semitic, the historic ā is preserved. Thus, an ancient Northwest Semitic language in which historic ā becomes ō can be classed as part of the Canaanite branch of Northwest Semitic.
Such features can be used as data of fundamental importance for the purposes of linguistic classification.
For example, a distinguishing feature of the ancient Old Hebrew script (i.e., Iron Age Old Hebrew script) is the fact that the letters bet, dalet, 'ayin, and resh do not have an open head, while Aramaic of the same period has open-headed forms. Similarly, the bet of Old Hebrew has a distinctive stance (namely, leans to the right), while the bet of the Aramaic and Phoenician script series has a different stance (namely, both of these lean to the left).
Recently, Christopher Rollston has suggested using the term isograph to designate a feature of the script that distinguishes it from a related script series (e.g., a feature that distinguishes the script of Old Hebrew from Old Aramaic and Phoenician, etc.).
The term isogloss (Ancient Greek ἴσος ísos "equal, similar" and γλῶσσα glōssa "tongue, dialect, language") is inspired by contour lines or isopleths such as isobar. However, the isogloss separates rather than connects points of equal language. Consequently, it has been proposed that the term heterogloss (ἕτερος héteros "other") be used instead.
See also 
- Fortson IV, Benjamin W. (2004). Indo-European Language and Culture. Blackwell Publishing. pp. 52–54. ISBN 1-4051-0316-7.
- Rix, Helmut (2001). Lexikon der indogermanischen Verben. Dr. Ludwig Reichert Verlag. p. 359. ISBN 3-89500-219-4.
- Garr, W. Randall (2 June 2008). Dialect Geography of Syria-Palestine: 1000-586 BCE. Eisenbrauns. ISBN 1-57506-091-4.
- Daniels, Peter; Bright, William, eds. (8 February 1996). The World's Writing Systems. New York: Oxford University Press. ISBN 0-19-507993-0.
- Rollston, Christopher A. (2006). "Scribal Education in Ancient Israel: The Old Hebrew Epigraphic Evidence". Bulletin of the American Schools of Oriental Research 344: 47–74.
- Sihler, Andrew L. (2000). Language History. Current issues in linguistic theory 191. Amsterdam/Philadelphia: John Benjamins Publishing. p. 170. ISBN 90-272-3698-4.
- Chambers, J.K.; Trudgill, Peter (28 December 1998). Dialectology. Cambridge Textbooks in Linguistics (2nd ed.). Cambridge University Press. ISBN 0-521-59646-7.
- Woodard, Roger D. (31 May 2004). The Cambridge Encyclopedia of the World's Ancient Languages. New York: Cambridge University Press. ISBN 0-521-56256-2.
- An example of an isogloss in Southern England.
- Beyond the Isogloss: The Isograph in Dialect Topography: A discussion of the shortcomings and oversimplifications of using isoglosses.
- On Some Acoustic Correlates of Isoglossy: A humorous analysis of Russian isoglossy.