People tend to use linguistic forms that were prevalent when they reached adulthood. So, in the case of linguistic change in progress, one would expect to see variation over a broader range of ages. William Bright provides an example taken from American English, where in certain parts of the country there is an ongoing merger of the vowel sounds in such pairs of words as 'caught' and 'cot'. This merger used to be distinctive of the western United States, but since World War II, it has developed independently in two other regions: western Pennsylvania and southwestward, and the New England coast from Boston north. 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.