In Tevye, a big red line (drawn by Tevye of course) separates Tradition with a capital T, which can never be disobeyed or disrespected; and things that are open to interpretation or even change: custom. See? It doesn't even get a capital letter. On the small-time custom side? Marriage practices, old-school values about money being the only indicator of success. On the Tradition side? Anything that takes you out of the Jewish culture and over to the side of the oppressive majority population.
Sholem Aleichem suggests that Tevye's reliance on Biblical quotations provides a sense of permanence and tradition in the face of constant and unpredictable change.
In Tevye, some of the village's traditions exist only in Tevye's mind.