The letters for consonants combine with diacritics for vowels. Hebrew uses the Hebrew alphabet with optional vowel diacritics. Impure abjads have characters for some vowels, optional vowel diacritics, or both. Other initial vowels use the a character modified by diacritics. Thai uses a combination of redundant consonants and diacritics. There are also a number of comma-like diacritics with "COMMA" in their Unicode names. Their spelling may contain diacritics, or accent marks. Some non-alphabetic scripts also employ symbols that function essentially as diacritics. These two diacritics originated separately; the diaeresis is considerably older. Over time, the ligatures may evolve into new letters or letters with diacritics. This includes Beninese Yoruba, which in Nigeria is written with both diacritics and digraphs. Acute, grave, and tilde diacritics are used to indicate pitch accents. Urdu also uses Persian names of the diacritics instead of Arabic names. The system includes four diacritics denoting tones. It also includes the most common diacritics and Arabic-Indic digits.