Although historically, languages were divided into three basic types (isolating, inflectional, agglutinative), the traditional morphological types can be categorized by two distinct parameters:
- morpheme per word ratio (how many morphemes there are per word)
- degree of fusion between morphemes (how separable the inflectional morphemes of words are according to units of meaning represented)
A language is said to be more isolating than another if it has a lower morpheme per word ratio.
To illustrate the relationship between words and morphemes, the English term "rice" is a single word, consisting of only one morpheme (rice). This word has a 1:1 morpheme per word ratio. In contrast, "handshakes" is a single word consisting of three morphemes (hand, shake, -s). This word has a 3:1 morpheme per word ratio. On average, words in English have a morpheme per word ratio substantially greater than one.
It is perfectly possible for a language to have one inflectional morpheme yet more than one unit of meaning. For example, the Russian word vídyat/видят "they see" has a morpheme per word ratio of 2:1 since it has two morphemes. The root vid-/вид- conveys the imperfective aspect meaning, and the inflectional morpheme -yat/-ят inflects for four units of meaning (third-person subject, plural subject, present/future tense, indicative mood). Effectively, it has four units of meaning in one inseparable morpheme: -yat/-ят.
Languages with a higher tendency toward isolation generally exhibit a morpheme-per-word ratio close to 1:1. In an ideal isolating language, visible morphology would be entirely absent, as words would lack any internal structure in terms of smaller, meaningful units called morphemes. Such a language would not use bound morphemes like affixes.
The morpheme-to-word ratio operates on a spectrum, ranging from lower ratios that skew toward the isolating end to higher ratios on the synthetic end of the scale. A larger overall ratio suggests that a language leans more toward being synthetic rather than isolating.
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