The letter which has t after or before it, besides referring to its own form, refers to those homogeneous letters which have the some prosodial length or time.,
By the previous sutra it was declared that a vowel standing by itself included all the letters of its class. Thus अ includes आ; and इ, ई; &c. This sutra lays down the rule by which the very form of the letter is taken and not all the letters of its class. This is done by placing a त् either after or before the letter. Thus अत् means the very letter अ and not all its homogeneous letters. Similarly उत् means short उ only and not long or protracted उ. We had to refer to this sutra, in our very first sutra, in explaining the term आत्.
The sutra consists of two words तपरः and तत्कालस्य. Taparah means that which has a त् after it or that which is after त्. Tat-kala means having the same time. According to time the vowels are divided into short, long and protracted. Short vowels have one measure, long vowels two, and protracted vowels three. A consonant lasts half the time of a short vowel. Therefore a letter preceded or followed by त्, refers to its own form as well as to those only of its homogeneous letters which have the same time. Thus the letter अत् will include the udatta, anudatta, svarita (both nasalised and non-nasalised) अ and not the long and protracted forms.
This sutra declares an injunction. The anuvritti of अण् of the previous sutra does not run into this sutra. Any other letter than अण् may be followed by त् and the rule of this sutra will apply to that also. This rule qualifies the previous sutra which would therefore mean that an अण् letter which is not followed or preceded by a त् includes its homogeneous letters also.
Thus in the sutra 7.1.9 अतोभिस ऐस्. 'After words ending in अत् (i. e. short अ), ऐस् takes the place of भिस्.' Thus वृक्षः. But in खट्वा which ends in long आ and whose prosodial time is different from अ, this rule will not apply, and we have खट्वाभिः.,