When ḍaṭ follows, thuk is the augment of the words 1. ṣaṭ 2. kati 3. katipaya and 4. catur,
The डट् is to be read into this sutra, and it should be changed into locative, as डटि 'when dat follows'. The word कतिपय is not a Numeral, it takes डट् by force of this sutra. Thus षण्णां पूरणो = षष्ठः 'sixth'. कतिथः 'the which in order?' कतिपयथः 'the one in order after several', चतुर्थः 'the fourth'.
Vart:- The affixes छ (ईय) and यत् come after चतुर् and there is elision of the first letter. As तुरीयः or तुर्यः ॥
In this and the two following sutras, the action of the ड् of डट् vanishes when an augment is added.,