The Greek script was the first documented writing system to represent consonants and vowels with distinct symbols, making it the oldest true alphabet. It has been used since the 8th century BC for writing the Greek language, and select letters are also currently used in writing mathematic and scientific concepts such as π (pi) and Ω (Ohm). Some symbols have been incorporated into the International Phonetic Alphabet. The script has also been used for writing a number of minority languages, including Urum, Albanian Tosk, and Balkan Gagauz Turkish. The Phoenician prince Cadmus is credited with introducing Phoenician writing to the Greeks, who then adapted it, most significantly by adding vowel letters, to write the Greek language. Historically, there were a number of variants of the alphabet, including an East/West variation. The Western variant was called Chalcidian, from which the Old Italic and Latin alphabets descended, and the Eastern variant was called Ionic, from which the modern Greek alphabet descended. A third variant, called the Attic script, was used in Athens until 400 BC, when they adopted the Ionic script. Subsequently the Ionic script became standard throughout Greece. At that time, Greek was written from right to left or in boustrophedon style, though it is now written from left to right. The alphabet currently contains twenty-four letters; six archaic letters have either been dropped or are now used as numerals. Some letters have variant forms. In writing the Greek language these variations are purely stylistic, although in mathematic and scientific contexts they can represent different concepts, for example ϖ, a variant form of pi, which is used in fluid dynamics to represent the angular frequency of a wave. The lower case sigma σ has a variant form, word-finally ς. Although this is a contextual form, it can be semantically significant; a word ending with σ must be an abbreviation of some other word having σ word-internally, whereas a word ending with ς must be a single non-abbreviated word. Historically, there were a number of ligatures used in Greek writing. Many of these have dropped from modern usage, although some remain, particularly in handwritten text and printed titles. The most common of these is the ligature ϗ̀, representing και, 'and'. There have also been three accents used with Greek vowel letters: acute (oxia), grave (varia) and circumflex (perispomeni). In the early 80s, the Greek Government instituted a language reform which, among other things, introduced a single diacritic (tonos) in place of the three accent system. When two vowels form a diphthong, the accent is placed on the second vowel. Breathing marks were historically used but have fallen from current usage. Languages other than Greek sometimes use other diacritics for representing sounds which are not present in Greek. Greek spelling employs a number of digraphs (pairs of letters used to represent a single sound), for example γγ which represents [ŋɡ] or [ɡ], even though a single γ represents [γ] / [j]. Greek punctuation provided the basis for Latin punctuation so the two systems are similar, although a Greek middle dot symbol represents a semicolon, and a Greek question mark looks like a Latin semicolon. Bold type is used in Greek writing for titles, but rarely within a body of text. Inside text, expanded letterspacing is often used.