The mysterious letters (muqaṭṭaʿāt, ḥurūf muqaṭṭaʿāt, "disjoined letters" or "disconnected letters") are combinations of between one and five Arabic letters figuring at the beginning of 29 out of the 114 chapters (surahs) of the Quran just after the Bismillāh Islamic phrase. The letters are also known as fawātiḥ () or "openers" as they form the opening verse of their respective surahs. Four chapters are named for their muqaṭṭaʿāt: Ṭā-Hā, Yā-Sīn, Ṣād and Qāf. The original significance of the letters is unknown. Tafsir (exegesis) has interpreted them as abbreviations for either names or qualities of God or for the names or content of the respective surahs. The general belief of most Muslims is that their meaning is only known to Allah. Inventory Muqatta'at occur in Quranic chapters 2–3, 7, 10–15, 19–20, 26–32, 36, 38, 40–46, 50 and 68. Multiple letters are written together like a word, but each letter is pronounced separately. Structural analysis There are 14 distinct combinations; the most frequent are ʾAlif Lām Mīm and Ḥāʾ Mīm, occurring six times each. Of the 28 letters of the Arabic alphabet, exactly one half appear as muqatta'at, either singly or in combinations of two, three, four or five letters. The fourteen letters are: ʾalif أ, hā هـ, ḥā ح, ṭā ط, yā ي, kāf ك, lām ل, mīm م, nūn ن, sīn س, ʿain ع, ṣād ص, qāf ق, rā ر. The six final letters of the Abjadi order (thakhadh ḍaẓagh) are unused. The letters represented correspond to those letters written without Arabic diacritics plus yāʿ ي. It is possible that the restricted set of letters was supposed to invoke an archaic variant of the Arabic alphabet modeled on the Aramaic alphabet. Certain co-occurrence restrictions are observable in these letters; for instance, ʾAlif is invariably followed by Lām. The substantial majority of the combinations begin either ʾAlif Lām or Ḥāʾ Mīm. In all but 3 of the 29 cases, these letters are almost immediately followed by mention of the Qur'anic revelation itself (the exceptions are surat al-ʻAnkabūt, ar-Rūm and al-Qalam); and some argue that even these three cases should be included, since mention of the revelation is made later on in the surah. More specifically, one may note that in 8 cases the following verse begins "These are the signs...", and in another 5 it begins "The Revelation..."; another 3 begin "By the Qur'an...", and another 2 "By the Book..." Additionally,