A phoneme is a contrast
A phoneme is a unit in a language's sound system that can distinguish words or grammatical forms. English pat and bat differ in initial /p/ and /b/ with the rest held constant: a minimal pair. [pʰ] in many English pronunciations of pin and [p] in spin are different phones, but their distribution depends strongly on position, so a standard analysis treats them as allophones of /p/. The pair of English words shows an English contrast; it does not determine whether an ancient Greek or Sanskrit stop was aspirated.
A minimal pair is strong evidence, not the only evidence. Near-minimal pairs, alternating morphemes, loan spellings and distributional restrictions can support an analysis. Conversely, two distinct spellings do not automatically equal two phonemes: scribes may alternate signs, or a script may distinguish conventions lost in speech. A good analysis records the word forms, meanings, date, dialect and exact environment.
Complementary distribution and free variation
Two phones are in complementary distribution if they occur in non-overlapping environments. That pattern suggests allophony when the sounds are phonetically related and no morphological or lexical complication explains the split. Free variation means a speaker may use more than one pronunciation in the same environment without a change in lexical meaning; even then, social or stylistic conditioning may exist. Neither category should be declared from two isolated words.
A practical table lists all occurrences of a candidate sound, with the preceding and following segments, syllable position, stress, morpheme boundary and source. Then look for a rule that predicts the distribution. A counterexample might reveal a different rule, a loanword, a dialect mixture or a faulty reading. Historical corpora are often too small to prove the absence of a contrast, so “not attested” is more honest than “impossible.”
Features and natural classes
Features describe recurring phonetic properties such as [voice], [nasal], [continuant], place and height. A process that affects /p t k/ but not /b d ɡ/ may refer to a class of voiceless stops. Features are analytic tools: the chosen theory and level of abstraction matter. A feature matrix is not a substitute for observed alternations or phonetic evidence.
Phonological rules model relationships between representations and surface outputs, often conditioned by environment. One conventional arrow notation, /n/ → [ŋ] / __ /k/, reads “a nasal becomes velar before /k/.” That is a schematic rule for learning notation, not a claim that every language has it. To establish a language-specific rule, supply actual tokens and test the predicted contexts.
Modern-language examples on this page clarify a linguistic pattern. They do not establish unattested sounds or forms for an ancient language. Follow the cited sources below and keep editorial readings separate from the surviving trace.