The affix ṭac is added to the words 1. rājan 2. ahan and 3. sakhi when standing at the end of a tat-puruṣa samāsa ,
Thus महाराजः, मद्रराजः, परमाहः, उत्तमाहः 6.4.145 राज्ञः सखा = राजसखः ॥ ब्राह्मणसखः ॥ Why not so here मद्राणां राज्ञी = मद्रराज्ञी instead of मद्रराजी ? For according to the maxim \a pratipadika denotes, whenever it is employed in Grammar, also such a nominal stem as is derived from it by the addition of an affix denoting gender\ (प्रातिपदिकग्रहणं लिङ्गविशिष्टस्यापि ग्रहणम्); the stem राजन् would denote the feminine also. To this we reply, that the construction of the sutra shows that the feminine राज्ञी is not to be taken. Because अहन् being a prosodially lighter word than राजन् ought to have been placed first in the compound. Its not being so placed indicates that that form राजन् only is to be taken whose final vowel becomes lengthened into आ in compounding with अहन्, i.e. राजा + अहन् ; and not राज्ञी + अहन् ॥,