Modern Greek is written in the Greek alphabet, which has 24 letters, each with a capital and lowercase (small) form. The letter sigma additionally has a special final form. There are two diacritical symbols, the acute accent which indicates stress and the diaeresis marking a vowel letter as not being part of a digraph. Greek has a mixed historical and phonemic orthography, where historical spellings are used if their pronunciation matches modern usage. The correspondence between consonant phonemes and graphemes is largely unique, but several of the vowels can be spelt in multiple ways. Thus reading is easy but spelling is difficult.