  The entanglement can be localized between two noncomplementary parts of a
many-body system by performing measurements on the rest of the system. This
localized entanglement (LE) depends on the chosen basis set of measurement
(BSM). We derive here a generic optimality condition for the LE, which, besides
being helpful in studying tripartite systems in pure states, can also be of use
in studying mixed states of general bipartite systems. We further discuss a
canonical way of localizing entanglement, where the BSM is not chosen
arbitrarily, but is fully determined by the properties of the system. The LE
obtained in this way, we call the localized entanglement by canonical
measurement (LECM), is not only operationally meaningful and easy to calculate
in practice (without needing any demanding optimization procedure), it provides
a nice way to define the entanglement length in many-body systems. For spin-1/2
systems, the LECM is shown to be optimal in some important cases. At the end,
some numerical results are presented for $j_1-j_2$ spin model to demonstrate
how the LECM behaves.
