  Localised defect modes generated by a finite line defect composed of several
masses, embedded an infinite square cell lattice, are analysed using the linear
superposition of Green's function for a single mass defect. Several
representations of the lattice Green's function are presented and discussed.
The problem is reduced to an eigenvalue system and the properties of the
corresponding matrix are examined in detail to yield information regarding the
number of symmetric and skew-symmetric modes. Asymptotic expansions in the far
field, associated with long wavelength homogenisation are presented. Asymptotic
expressions for Green's function in the vicinity of the band edge are also
discussed. Several examples are presented where eigenfrequencies linked to this
system and the corresponding eigenmodes are computed for various defects and
compared with the asymptotic expansions. The case of an infinite defect is also
considered and an explicit dispersion relation is obtained. For the case when
the number of masses within the line defect is large, it is shown that the
range of the eigenfrequencies can be predicted using the dispersion diagram for
the infinite chain.
