The words pāra 'across' and madhya 'middle' may optionally be compounded with a word ending in the 6th-Case-affix, when they take forms pare and madhye and the compound so formed is avyayībhāva,
Ordinarily these words would have formed Possessive Tatpurusha compounds. The present sûtra ordains Avyayibhåva instead. The force of वा in the text is that the Genitive Tatpurusha compounding also takes place in the alternative. As पारेगङ्गम् or गङ्गापारम् 'across the Ganges.' मध्येगङ्गम् or गङ्गमध्यम् 'middle of the Ganges.'
Of course according to the option allowed by sutra 2.1.11, this compounding need not take place at all, the same sense being expressed by a phrase; as पारं गङ्गायाः or मध्यं गङ्गायाः .,