आदिरिति द्व्यक्षरं प्रतिहार इति चतुरक्षरं तत इहैकं तत्समम् ॥ २.१०.२ ॥
ādiriti dvyakṣaraṃ pratihāra iti caturakṣaraṃ tata ihaikaṃ tatsamam || 2.10.2 ||
2. The word ādi is two-syllabled, and the word pratihāra is four-syllabled. If you take away one syllable from pratihāra and add it to ādi, then they will have the same number of syllables.
Word-for-word explanation:
Ādiḥ iti dvi-akṣaram, the word ādi is two-syllabled; pratihāraḥ iti catuḥ-akṣaram, the word pratihāra is four-syllabled; tataḥ, from that [i.e., from the word pratihāra]; ekam, [take away] one [syllable]; iha, [and add] here [to the word ādi]; tat samam, that makes them equal [both three-syllabled].
Commentary:
Om is the ādi of the sevenfold Sāma. It is ādi (the beginning) because a person begins singing the Sāma with Om.