साम आकम्

आकम् is substituted for the Genitive 6th-Case plural affix साम् -- आमि सर्वनाम्नः सुट् 7.1.52, after the अङ्ग (stems) युष्मद् and अस्मद्।
साम् is the affix आम् of the Genitive plural with the augment स ॥ Thus युष्माकम् and अस्माकम् ॥ Why is it read साम् and not आम्. when there is no स् at the time when the substitution is ordained? It is read as साम् in order to indicate that आकम् will not get the augment स्, for otherwise 'yushma' and asma' having lost their 'd' by 7.2.90, end in अ, and so by 7.1.52, would cause the genitive affix to get the augment स; the present sutra removes that also. The substitute is exhibited with a long आ, in order to make अ + आ = आ in युष्म + आकम्, had it been short अ, then there would have been no lengthening but अ + अ = अ by 6.1.97. If you say 'the very fact that अकम् was taught and not कम्, would prevent para-rupa and cause lengthening'; we reply, that the अ of अकम् would find its scope in preventing ए substitution. For without अ, we should have युष्म + कम् = युष्मेकम् 7.3.103.