In Somali, the tone-bearing unit is the mora rather than the vowel of the syllable. A long vowel or a diphthong consists of two morae and can bear two tones. Each mora is defined as being of high or low tone. Only one high tone occurs per word and this must be on the final or penultimate mora. Particles do not have a high tone. (These include prepositions, clitic pronouns for subject and object, impersonal subject pronouns and focus markers.) There are therefore three possible "accentual patterns" in word roots.