आद्यन्तवदेकस्मिन्

An operation should be performed on a single letter, as upon an initial or upon a final.
This is an atidesa sutra and consists of four words, आदि "the beginning," अन्त "end," वत् "like," एकस्मिन् "in one;" the literal translation being "beginning is end-like in one." The affix वत् has the force of the locative, and the meaning is "on the beginning," and "on the end," i.e., let an operation be performed on a single letter, as upon an initial or upon a final. Thus by 3.1.3. an affix has udatta or acute accent on its initial vowel. Thus the affix tavya has accent on the first अ, and we have कर्त्तव्य॑म्. But when an affix consists of a single vowel, it is itself the beginning and the end, and, as such, will have the accent. As in औपगवम् aupagavam the last a has the accent, which a represents the affix अण्.

Similarly, by 7.3.102, the final of a base ending in short अ is lengthened before a case-affix beginning with a consonant of यञ् class. Thus राम + भ्याम् = रामाभ्याम्. This rule will apply when the word consists of one letter only, and that letter is a short अ. Thus अ + भ्याम् = आभ्याम् "by those two." Here it must not be objected that the solitary अ is initial, and cannot be final.

Why do we say "in one"? If a word consists of more than one letter, then the beginning is not like the end. Thus in the word सभा the initial short अ is not like the final long आ; otherwise सभा would get the name of वृद्धम् 1.1.73, and the secondary derivative from सभासन्नयन would be formed by the affix छ् 4.2.114. But this is not so, the affix अण् is employed instead, and the form is साभासन्नयनः.