The affix ṣṭarac (tara with feminine -- anyato ṅīṣ [[4.1.40]]) comes in expressing diminution after the words 1. kāsū and 2. goṇī,
Thus हृस्वा कासूः = कासूतरी 'a small lance'; गोणीतरी 'a small sack'.,
कासू-गोणीभ्यां ष्टरच्
Adhyāya 5 · Pāda 3 · Rule 90