ईदूदेद्द्विवचनं प्रगृह्यम्

A dual case affix ending in ई or ऊ or ए is called प्रगृह्य , or excepted vowels which do not admit सन्धि or conjugation.
As a general rule, Sanskrit allows of no hiatus in a sentence. If a word ends in a vowel, and the next word begins with a vowel, the two vowels coalesce, according to certain rules. This is called sandhi. But pragrihyas are exceptions to this sandhi, "they are certain terminations, the final vowels of which are not liable to any sandhi rules." 6.1.125. This sutra gives three of these terminations, viz., the duals of nouns or verbs ending in ई (ईत्), ऊ (ऊत्), ए (एत्). Thus:- कवी इमौ "these two poets," so also वायू इति "two winds;" माले इति "these two garlands." पचेते इति "they two cook" पचेथे इति "you two cook." Here according to the general rule of sandhi, the final इ of कवी and the initial इ of इमौ ought to have coalesced into an ई, but it is not so, because कवी is the nominative dual of कवि.

Why do we say ending in ī, ū and e? Because when the dual ends in any other vowel this rule will not apply. As वृक्षौ+अत्र = वृक्षावत्र, "these two trees" 6.1.78. Of course it follows from the definition, that ई, ऊ, or ए of the dual number only are pragrihya; if these vowels terminate any other number they will not be pragřihya but will follow the general rule of sandhi, as कुमारी (I.S.) + अत्र = कुमार्य्यत्र, "the girl is here" 6.1.77.

Vart: The prohibition of मनीव &c., should be stated in treating of the pragrihya nature of ई, ऊ &c. Thus मनी (two gems) + इव = मनीव "like two gems". So also दम्पतीव "like a couple," जम्पतीव "like man and wife," रोदसीव "like heaven and earth." Some say that this vårtika is unneccessary. Because the final member of the above words is not इव, but the particle व only; which has the same meaning as iva.