  A knot theory for two-dimensional square lattice is proposed, which sheds
light on design of new two-dimensional material with high topological numbers.
We consider a two-band model, focusing on the Hall conductance {\sigma}xy =
e^2/hbar*P, where P is a topological number, the so-called Pontrjagin index. By
re-interpreting the periodic momentum components kx and ky as the string
parameters of two entangled knots, we discover that P becomes the Gauss linking
number between the knots. This leads to a successful re-derivation of the
typical P-evaluations in literature: P = 0;{\pm}1. Furthermore, with the aid of
this explicit knot theoretical picture we modify the two-band model to achieve
higher topological numbers, P = 0;{\pm}1;{\pm}2.
