Minimal pairs

An important question which may have occurred to you already is: how can we tell what is a phoneme? One of the most robust tools for examining phonemes is the minimal pair. A minimal pair is a pair of words which differ only in one segment. For example, the English words do /du/, too /tu/, sue /su/, moo /mu/ all form minimal pairs with each other. In a minimal pair one can be sure that the difference between the words is phonemic in nature, because the segments in question are surrounded by the same environment and thus cannot be allophones of each other.

This is not a foolproof tool. In some cases it may by chance be impossible to find a minimal pair for two phonemes even though they clearly contrast. In many cases it is possible to find near-minimal pairs, where the words are so similar that it is unlikely that any environment is conditioning an allophone.

Finally this also requires some common sense, since phonemes may be in complementary distribution without being likely allophones. For instance, the English phonemes /h/ and /ŋ/ (both occurring in the word hung /hʌŋ/) can never occur in the same environment, as /h/ is always syllable-initial and /ŋ/ always syllable-final. However few would suggest that these phonemes are allophones. Since English speakers never confuse them, they are auditorily quite different, and substituting one for another in a word would render it unintelligible. Unfortunately there is no hard-and-fast consensus on precisely how to be sure sounds are allophones or not, and in many languages there is vigorous debate.