We encounter more complex conditions when a substituent is introduced on one of the carbon atoms of a monomer and proceed from polyethylene to vinyl-type polymers. The first type of isomerism which presents itself is of the positional type and involves the arrangement of the substituents concerning the number of carbon atoms at which they are attached as one travels along the length of the chain. The two pure cases are: the head-to-tail arrangement and the head-to-head, tail-to-tail arrangement. Both arrangements involve a definite sequence of the substituents over the entire length of the chain.