तदर्थं विकृतेः प्रकृतौ

After the word being the name of a product, the above-mentioned affix comes, to denote a thing which is the primitive that is serviceable for that.
The word प्रकृति means 'primitive or the material cause of a product', The word विकृति means 'the product' or 'modification of such primitive' तदर्थे 'serviceable for that' i. e. 'the primitive being serviceable for the product'. The word तदर्थे shows that the primitive must be serviceable for that particular product and nothing else. That is to say, an affix will not come to denote any product in general, but only when the primitive is serviceable for a particular product and nothing else. The word तदर्थ shows also that the word must be in the 4th case, when the affix is added. Some read the words तस्मै हितं also into this sutra.

Thus अङ्गारेभ्यो हितानि एतानि काष्ठानि = अङ्गारीयाणि काष्ठानि 'wood serviceable for making charcoal;' so also प्राकारीया इष्टकाः 'bricks serviceable for making a wall'. Here अङ्गारीय and प्राकारीय are formed by छ 5.1.1. Similarly शङ्कव्यं दारु 'wood serviceable for a spear.', पिचव्यः कार्पासः 'cotton plant serviceable for cotton'.

Why do we say तदर्थम् 'serviceable thereto'? Observe यवानां धानाः, धानानां सक्तवः ॥ Here there is modification of primitive, but सक्तु is the product also of other things than धानाः such as लाजा &c.

Why do we say विकृतेः 'after a product'? Observe उदकार्थः कूपः, there is no affix. For कूपः 'a well' and उदक 'water' do not stand in the relation of and प्रकृति and विकृति i.e. material cause and effect. As 'water' is not the modification or product of 'the well, though it is found in the well, and therefore, the affix is not employed.

Why do we say प्रकृतौ 'to denote a primitive'. Observe अस्यर्था कोशी 'a sheath serviceable for a sword'. The sword is a product of iron, but a sheath is not the primitive of the sword.

By using the words प्रकृति and विकृति it is further shown that the prakriti is liable to modification or vikara.