In a aṅga (stem) ending in n the preceding vowel is lengthened in strong cases with the exception of the Vocative Case singular.,
As राजा, राजानौ, राजानः, राजानम्, राजानौ ॥ So also सामानि तिष्ठन्ति ॥ राजा was राजान्स्, the स् was elided by (VI. I. 68), and म् by 8.2.7. Why do we say 'in strong cases'? Observe राजनि (Locative Singular); सामनि (Locative Singular). Why do we say 'but not in the Vocative Singular'? Observe हे राजन् ! हे तक्षन् !,