After a aṅga (stem) ending in a are substituted in for the Instrumental -3rd case ending; ā (āt) for the Ablative -5th case ending as and sya for the Genitive 6th-Case ending as,
Thus वृक्षेण and प्लक्षेण; वृक्षात् and प्लक्षात्; and वृक्षस्य and प्लक्षस्य ॥ After stems not ending in अ we have the original ending, as पत्या, सख्या ॥ Some would have the forms अतिजरसिन and अतिजरसात् from अतिजर ॥ This is on the analogy of अतिजरसैः [See Sutra 9 ante]. Thus अति + जरा = अतिजर 1.2.48. Then by 7.2.101, जरस् is substituted for जर before the vowel-affixes. These forms are not supported however by Patanjali. According to him the forms will be either अतिजरेण, अतिजरात्, or अतिजरसा and अतिजरसः but never अतिजरसिन and अतिजरसात् ॥,