ससूपाणाम् एकशेष एक-विभक्तौ

This is a very important sutra, and deals with what is technically called Ekasesha (or retention of one). When there are two or more words of the same form, and the same case termination, standing together, one is retained and the rest are dropped. Thus वृक्षः + वृक्षः = वृक्षौ (trees, in dual) वृक्षः + वृक्षः + वृक्षः = वृक्षाः.

Every individual thing requires an individual word to express it; one word therefore could not express many things; to prevent the repetition of the same word to express many objects of similar form, is the purpose of this sutra, by which one word is only retained.

This applies only to words having the same form and not if they have different forms; as प्लक्ष न्यग्रौधाः the plaksha and nyagrodha trees.

The word \form\ is used in the text to show that even if two words have different meanings, but the same form, the rule of Ekasesha will apply, as अक्षः (die) + अक्ष (eye) + अक्षः (axle) = अक्षाः. Similarly पादाः and भाषाः.

The present rule only applies to the retention of one, and prevents the retention of two or more.

The word शेष (retention) is used to show that there is not a substitution (adesa) of one for many, but the retention of one out of many.

The rule will not apply if the two words are in different case-terminations, though they may have the same form. As पयः (1. s.) पयो (2. s.) जरयति ब्राह्मणाभ्यां (3 d.) च कृतं ब्राह्मणाभ्यां (4. d.) च देहि.,