Hence it is the _Atharva Sikhá Upanishad_ in which the investigation of the sacred word _Om_ is principally conducted apart from those of the _Mándukya_, _Maitrí_ and _Táraka_ Upanishads. (Web. Id., p. 164). These together with their Bháshyas by Sankara, the Kárikás of _Gaudapáda_, and the commentaries of _Ánandagiri_ on them, are chiefly devoted to the scrutiny of the sacred syllable, beside the partial disquisition of every other Upanishad and theological work into the hidden sense of this mystic word. Weber points out the Saunaka and Pranava Upanishads among the number (A. S. L., p. 165). [Sidenote: 7. Mode of our Investigation.] We shall proceed in this prolegomena first to investigate into the orthographical character and structure of this syllable, and then to inquire into the designations and etymological synonyms or the word, with the lexical meanings that we can get of them, and lastly to treat of the many mystical interpretations which this single word is made to bear as a common emblem of them. II. ORTHOGRAPHY OF OM. [Sidenote: 1. The letter ॐकारः] Firstly: Om with respect, to its name and utterance is called Onkára, that is, the nasal _On_ in combination with the adjunct kára (signifying a sound) and meaning the letter _On_. For all sounds whether vocal (स्वरबर्ण) or sonant (व्यञ्जन), nasal (सानुनासिक) or not-nasal (निरनुनासिक), articulate (ब्यक्तस्फुट) or onomatopoeia (अब्यक्तास्फुट), are denominated letters; as the letters _a_ &c. (अकारादि) are called vowels, the letters Ka &c. (ककारादि) consonants; so the nasals _Án_, _in_ (आं इँ) &c., as also the inarticulate ones (फङ्कार) &c., are all letters; but the Onkára is the root of all; thus ॐकारेण सर्ब्बबाक् सस्तणोति । माण्डुक्योपनिषद् । Manu calls it a letter in the passage:—“This one letter is the emblem of the Most High”. II.83. _Vide_ Dr. Mitra’s Ch’hánd Up, p. 4. [Sidenote: 2. A conjunct Letter युक्ताक्षरः] But here a question is raised as to whether a conjunct vowel or consonant may with propriety be styled a single letter or not. To this says Dr. R. L. Mitra in a foot-note to his translation of the _Ch’hándogya_ Upanishad that—“It is true that this emblem conveys two sounds, that of _O_ and _m_, nevertheless it is held to be one letter in the above sense; and we meet with instances even in the ancient and modern languages of Europe that can justify such privileges, such as _xi_ and _psi_, reckoned single letters in Greek, and Q. W. X. in English and others.” (Ch 1. Sec. 1. p. 4). So is _lámálif_ in Persian &c. The Sanskrit conjunct _ksha_ (क्ष) is considered a single consonant, when they say, ककारादि क्षकारान्तं ब्यञ्जनं परिकीर्त्तितं । [Sidenote: 3. The Syllable Om ओमक्षरं ।]