Isogloss
A boundary line on a map showing where a particular linguistic feature changes. Isoglosses reveal dialect boundaries.
Real World
The 'bath–trap split' isogloss runs roughly across the Midlands, separating southern English speakers (long /ɑː/ in 'bath') from northern speakers (short /a/).
Exam Focus
Use 'isogloss' precisely — it marks a single feature boundary; reference 'bundle of isoglosses' when describing stronger dialect zones.
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