sentences:  Examining the speech across several generations of a single family, one would find the grandparents' generation would never or rarely merge these two vowel sounds; their children's generation may on occasion, particularly in quick or informal speech; while their grandchildren's generation would merge these two vowels uniformly.This is the basis of the apparent-time hypothesis where age-based variation is taken as an indication of linguistic change in progress.A commonly studied source of variation is regional dialects (regiolects).