Gāhān is the oldest Iranian poem in the Avestan language. In this research, the external, sidelong and internal music of Gāhān are investigated. The data collection method is library-documentary and analytical research method. The study of ancient literary texts based on new literary criticism leads to a better understanding of them as the origin of Persian literature. The  external music surveys the metre. Gāhān has syllabic order. in sidelong music rhyme is explored, Gāhān has internal rythme. in internal music the repetition stylistics are so importante which are divided into visible and invisible. The  invisible repetitions used in Gāhān are redundant puns, subsequent pun, derivation and pseudo-derivation. Also single-place alliteration and multi-place alliteration are used in Gāhān. Gāhān’s Visible repetitions are repetition of words during a verse, regular repetition of one or more words at the beginning of verses, repetition of stanzas during a poetic form, repetition of one or more words sporadically in The length of a few verses, the regular repetition of one or more words at the beginning of the verses, and red Al-ajoz ala Al-sadr (Anadiplosis). endings  and declension, matching adjectives and pronouns with noun, prefixes and suffixes, repeating conjunctions, prepositions and adverbs also create music.
Keywords: old Iranian poetries, Gāhān, poem’s music, external music, internal music, repetition.
Introduction: poetry and music have a unbreakable connection. The set of factors that take words out of their normal and simple form and give them an artistic effect is called poetry music such as types of metre, rhymes, radif, sound harmonies such as types of puns and … Poetry music includes four categories: 1- External music, 2- Sidelong music, 3- Internal music, and 4- Spiritual music. The  external music surveys the metre. in sidelong music rhyme is explored and in internal music the repetition stylistics are so importante. In this research, some external, Sidelong, and internal music components of Gāhān, are investigated, and the investigation of the spiritual music of the hymn is not possible due to the length of the article, and it can be the subject of a separate article. The aim of the research is to examine Gāhān as a collection of ancient poems based on musical features and literary techniques found in New Persian poems. Also, investigating the age of the use of these Stylistic Devices and how and their frequency in Gāhān as the oldest Iranian poetry, is another goal of this research.
Materials & methods: The data collection method is library-documentary and analytical research method.
Discussion and results Gāhān is the oldest Iranian poem that remains in the Avestan language, and its composer is Ashu Zoroaster, whose approximate life time dates back to 1000 BC. Gāhān has 17 hāts, which are located between yasnas and includes five Gāhs. Gāhān has syllabic order and  internal rhyhme. Rhyme in Avestan hymns is in the form of incomplete puns and a kind of sound balance and harmony. In internal music the repetition stylistics are so importante which are divided into visible and invisible. The  invisible repetitions used in Gāhān are redundant puns, subsequent pun, derivation and pseudo-derivation. Also single-place alliteration and multi-place alliteration are used in Gāhān. All kinds of puns have been used 82 times in Gāhān, and the highest frequency is related to derivational puns and the lowest frequency is related to pseudo-derivative puns. The highest frequency of the use of puns among the times is related to Ahunavaitī gāθā and the lowest is related to Vohu xšaϴrā gāϴā. All kinds of Alliteration (except Hampaghazi), which is considered as a stylistic and linguistic feature of Gāhān are sometimes found with high frequency. Gāhān’s Visible repetitions are repetition of words during a verse, regular repetition of one or more words at the beginning of verses, repetition of stanzas during a poetic form, repetition of one or more words sporadically in The length of a few verses, the regular repetition of one or more words at the beginning of the verses, and red Al-ajoz ala Al-sadr (Anadiplosis). Types of visible repetitions are used 69 times in Gāhān. In the meantime, the most usage is related to the repetition of the word during one verse and the least usage is related to the Anadiplosis. The highest frequency of visible repetitions in  Gāhān is related to Ahunavaitī gāθā and the lowest frequency is related to spəntā mainyū gāϴā. In Gāhān, Ahunavaitī gāθā had the highest frequency of using stylistic devices, and spəntā mainyū gāϴā and Vohu xšaϴrā gāϴā had the lowest frequency. endings  and declension, matching adjectives and pronouns with noun, prefixes and suffixes, repeating conjunctions, prepositions and adverbs also create music. It can be said that one of the reasons for phonetic harmony at the end of some Avestan words is the special grammatical rules of this language. One of the features of Avesta language is the use of nouns and adjectives in eight institutional forms, nominative, accusative, instrumental, dative, ablative, genitive, locative, and vocative.