Free variation

In chapter 6 we noted that some speakers of English substitute a glottal stop for the [t] at the end of words such as don't or can't or in the middle of words such as bottle or button. The substitution of the glottal stop does not change the meaning; [do~ʊnt] and [do~ʊnʔ] do not contrast in meaning, nor do [bɒtəl] or [bɒʔəl]. A glottal stop is therefore not a phoneme in English since it is not a distinctive sound. These sounds, [t] and [ʔ], are in free variation in these words.

Similarly at the and of a words, a stop consonant may be released or unreleased. That is, it will not change the meaning of the word rope if in pronouncing it, you keep your lips together or open them. An unreleased stop is transcribed phonetically with the diacritic [1] after it, as [roʊp1]. Thus in English unreleased stops do not contrast with released stops. Released and unreleased stops occur in free variation. You may freely use one or the other. Released stops are not distinct phonemes in English.

There is another way in which two sounds may be in free variation. In English a substitution of [i] foe [e] in economics does not change the meaning of the word. Some speakers pronounce the word with an initial [i] and other with an initial [e]. However, [i] and [e] are not in free variation in all words, since we cannot substitute [i] and [e] foe each other in other words, such as beat [bit] and bet [bet]. Did you beat the drum ? does not mean the same thing as Did you bet the drum ? Asong of the 1930s was based on the notion of free variation  :