deriv SD cv (266) ashtadhyayi.com hei.de L 266 ETT STT a 7.1.25 ALPH OLDHOMEPAGE NEWHOMEPAGE

aD DatarAdibhyaH paJcabhyaH

अड्डतरादिभ्यः पञ्चभ्यः ONPANINI 71025

(Then replace that /am) with at अत् after (neuter) /Datara Datama डतम itara इतर /anya- anyatara अन्यतर.

anya- अन्यॱ n + /su OR /am
→ anya- अन्यॱ + /am by < atom
→ anya- अन्यॱ + at अत् by this rule
→ !**anyat अन्यत् by amipUrvaH, because ad अद् is /sthAnivat.

Does the ad अद् in the sUtra represent ad अद्, or at अत्?

No one knows, but as it ends up being padafinal, it does not matter. Spell it as ad अद् if u like.

Why does my book spell this sUtra as adD DatarA अद्ड्डतरा?

Long story. See Kiparsky on markers for details.

Anyway, however it is spelt, it is always chanted as this rule. You MUST chant like that, because rule saMyogAntasy... turns adDDa अद्ड्ड into AdDa आद्ड, then STunASTuH turns that into aDDa अड्ड.