In the science of Glottology
: a way of representing local usage in pronunciation
or word choice
. The isogloss
is a line drawn on a map that encloses the region where a certain linguistic phenomenon happens.
For example, in standard Italian the concept of "now" can be expressed by two, perfectly equivalent words:ora and adesso.
Interestingly, ora is used in regions south of Tuscany and Marche. You can thus draw an isogloss that roughly follows the Appennines and cuts Italy in two: north of it, adesso rules, and south of it ora is used.
Who can draw me the isoglosses for: y'all, youse and you guys ?
By studying the isoglosses for various sets of linguistic phenomena, you can trace migrations, trade routes and linguistic innovation over the centuries.
sincere thanks to Girlface for the nodeshell