आर्हादगोपुच्छसंख्यापरिमाणाट्ठक्

Up to sutra 5.1.63, inclusive, the affix ठक् (+/- इक or +/- क)
Karika

ऊर्ध्वमानं किलोन्मानं परिमाणं तु सर्वतः ।
आयामस्तु प्रमाणं स्यात्संख्या बाह्या तु सर्वतः ॥

Strictly speaking ठक् governs upto sutra 5.1.65. The word आर्हाद् is made up of आ + अर्हात् The force of आ here is अभिविधि or inclusive, so that in the Sutra तदर्हति 5.1.63 also, the ठक् will be employed. The affix ठक् debars ठञ् ॥ The sense will be the same as given in 5.1.37 &c. the difference between ठक् and ठञ् being in accent only. Thus निष्कया क्रीतं = नैष्किकम्, So also पाणिकम् ॥

But गोपुच्छेन क्रीतम् = गोपुच्छिकम्, the accent falling on गौ; so also with words denoting Numbers and Measures, as षाष्टिकम्, प्रास्थिकम्, कौडविकम् ॥

The word परिमाण in this sutra has its technical signification, and not its etymological sense. Technically परिमाण does not include संख्या, etymologically it does. Panini has used this word परिमाण in some sutras in its technical sense, in others in its etymological sense. Thus in sutras 4.1.22, and 5.1.19 the technical sense is taken, while in sutras 4.3.156 and 5.1.57 the etymological sense is meant. Technically परिमाण means सर्वतो मानं or a measure of capacity, and surface namely length and breadth and height or depth, such as प्रस्थः &c, i.e. a cubic measure and a square measure. In this sense it is to be distinguished from the measures of length or lineal measure, technically called प्रमाण, such as a वितस्त्य 'a cubit' &c, on the one side, and the measures of weight, technically called उन्माण such as पल, &c on the other side. In short उन्मान means a measure of weight or gravity, परिमाण means a measure of volume or capacity, प्रमाण means a lineal measure, and a संख्या is beyond all these.