What Does Isogloss Mean in Linguistics?

An isogloss is a geographical boundary line marking the area in which a distinctive linguistic feature commonly occurs. Adjective: isoglossal or isoglossic. Also known as heterogloss. From the Greek, "similar" or "equal" + "tongue". Pronounced I-se-glos.

This linguistic feature may be phonological (e.g., the pronunciation of a vowel), lexical (the use of a word), or some other aspect of language. 

Major divisions between dialects are marked by bundles of isoglosses.

Examples and Observations

• "[S]peakers in southern Pennsylvania say bucket, and those in the north part of the state say pail. [The line of demarcation between the two] is called an isogloss. Dialect areas are determined by large 'bundles' of such isoglosses.
  "Several noteworthy projects have been devoted to mapping the features and distribution of dialects across the United States, including Frederic Cassidy's Dictionary of American Regional English [DARE] (begun in the 1960s and [completed in 2013]), and William Labov, Sharon Ash, and Charles Boberg's The Atlas of North American English (ANAE), published in 2005."
• Regional Dialects
  "English is made up of a number of regional dialects... Linguists can identify the main characteristics of different regions, and the isoglosses establish boundaries which group together non-standard dialect forms with similar distinctive linguistic features. Inevitably, there are some overlaps--although non-standard lexis tends to be located in specific regions, non-standard grammatical features are similar across boundaries."
• Drawing an Optimal Isogloss: 
  "The task of drawing an optimal isogloss has five stages:
  • Selecting a linguistic feature that will be used to classify and define a regional dialect.
  • Specifying a binary division of that feature or a combination of binary features.
  • Drawing an isogloss for that division of the feature, using the procedures described below.
  • Measuring the consistency and homogeneity of the isogloss by the measures to be described below.
  • Recycling through steps 1-4 to find the definition of the feature that maximizes consistency or homogeneity."
• Focal Areas and Relic Areas
  "Isoglosses can also show that a particular set of linguistic features appears to be spreading from one location, a focal area, into neighboring locations. In the 1930s and 1940s Boston and Charleston were the two focal areas for the temporary spread of r-lessness in the eastern United States. Alternatively, a particular area, a relic area, may show characteristics of being unaffected by changes spreading out from one or more neighboring areas. Places like London and Boston are obviously focal areas; places like Martha's Vineyard--it remained r-pronouncing in the 1930s and 1940s even as Boston dropped the pronunciation--in New England and Devon in the extreme southwest of England are relic areas."
• Kinds of Linguistic Features
  "Further distinctions can be made in terms of the kind of linguistic feature being isolated: an isophone is a line drawn to mark the limits of a phonological feature; an isomorph marks the limits of a morphological feature; an isolex marks the limits of a lexical item; an isoseme marks the limits of a semantic feature (as when lexical items of the same phonological form take on different meanings in different areas)."
• The Canadian Shift Isogloss
  "A given region may have optimal conditions for a given sound change, which may affect almost all speakers. This is the case with the Canadian Shift, involving a retraction of /e/ and /ae/ . . .; it is especially favored in Canada because the low back merger that triggers the shift takes place well to the back of the vowel space for almost everyone. Homogeneity for the Canadian Shift isogloss, which stops at the Canadian border, is .84 (21 of the 25 speakers within the isogloss). But the same process takes place occasionally throughout other areas of low back merger in the U.S., so that consistency for the Canadian isogloss is only .34. Outside of Canada, the instances of this phenomenon are scattered throughout a much larger population, and leakage is only .10. Homogeneity is the crucial measure for the dynamics of the Canadian vowel system."

Sources

• Kristin Denham and Anne Lobeck, Linguistics for Everyone: An Introduction. Wadsworth, 2010
• Sara Thorne, Mastering Advanced English Language, 2nd ed. Palgrave Macmillan, 2008
• William Labov, Sharon Ash, and Charles Boberg, The Atlas of North American English: Phonetics, Phonology, and Sound Change. Mouton de Gruyter, 2005
• Ronald Wardhaugh, An Introduction to Sociolinguistics, 6th ed. Wiley-Blackwell, 2010
• David Crystal, A Dictionary of Linguistics and Phonetics, 4th ed. Blackwell, 1997
• William Labov, Sharon Ash, and Charles Boberg, The Atlas of North American English: Phonetics, Phonology, and Sound Change. Mouton de Gruyter, 2005