Pawn chain - definition by Nimzowitsch vs modern books

Dear Chess Players,

Aron Nimzowitsch in his book "My system" defined the base of the pawn chain by the following way: it is the last "BLOCKED" pawn (if we look backward) of the pawn chain. He paid attention on fact that attack should be directed to the last blocked pawn. In modern chess books (IM Silman's, GM Seirawan's or in today internet sites) it is written that the base of the pawn chain is the last unprotected pawn (not necessarily blocked). It is obvious, that if chess player have many pawn chains consists of two pawns in which last is unblocked, he should protect unblocked pawns because he can loose the material, but it is not connected with blockade ideas. My question is the following: Is the modern definition of a pawn chain new point of view? Have somebody done new discovery about which I haven't know?

Nimzo had it right, and the others are probably just sloppy. The base must be fixed (blocked) before it can be attacked.