As shown in Figure 3074a, a two-dimensional (2-D) lattice has the following characteristics:
        i) It has two non-collinear basis vectors (a and b). The interaxial angle γ determines the relationship between the two basis vectors.
        ii) A translation of na + pb (e.g. 1a + 3b in Figure 3074a) from one lattice point to another must form an “identical” lattice structure. Here, n and p are  integers.

The number of the lattice points per unit cell in 2-D lattices can be given by,
         ---------------------------- [3074]
where,
          N_Interior and N_Corner - The numbers of the lattice points inside the unit cell and at the corners, respectively, as shown in Figure 3074b.

Figure 3074b. Lattice points inside the unit cell and at the corners in 2-D lattices.

In 2-D lattice, there are five distinct Bravais lattices. In this case, there are only four crystal systems:
        i) Oblique.
        ii) Rectangular.
        iii) Hexagonal.
        iv) Square.