The Sino-Tibetan languages have in common several features, which are exhibited to a greater or lesser extent in the individual tongues. For example, they show a tendency to be monosyllabic and isolating and to use tones or musical pitch. In an isolating language the words do not change their form or show inflection. Because of the relative absence of inflection, word order is the key to expressing grammatical relationships. A monosyllabic language has a limited number of syllables since the sound combinations that are possible are also limited in number. Because there are so many words that sound alike, two words of similar meaning are often used together to make the sense clearer. Combinations of two or more monosyllabic words also increase the vocabulary. Classifiers, which vary according to the sense of the words with which they are used, aid in making root meanings clear. For instance, one classifier is employed with round articles, and another with items of clothing. The use of different tones for each monosyllable has two striking benefits. It increases the vocabulary by multiplying the number of possible monosyllables, and it also is helpful in distinguishing among homophones. The number of tones differs in each language; three tones are found in Burmese, five in Thai, four in Mandarin Chinese, and nine in Cantonese Chinese.