The lattice method is a written method of multiplying numbers that uses a lattice to break down the process into smaller steps. It is also known as the Italian method, Chinese method, Chinese lattice, gelosia multiplication, sieve multiplication, shabakh, diagonally or Venetian squares. The method has been used for centuries in many different cultures and is still being taught in certain curricula today.

To use the lattice method, a lattice is first constructed, sized to fit the numbers being multiplied. The multiplicand is placed along the top of the lattice so that each digit is the header for one column of cells, and the multiplier is placed along the right side of the lattice so that each digit is a (trailing) header for one row of cells. Diagonal lines are then drawn in the lattice to bisect each cell, and the numbers in each diagonal are summed. The final product is composed of the digits outside the lattice which were just calculated.

The lattice technique can also be used to multiply decimal fractions. For example, to multiply 5.8 by 2.13, the process is the same as to multiply 58 by 213 as described in the preceding section. To find the position of the decimal point in the final answer, one can draw a vertical line from the decimal point in 5.8, and a horizontal line from the decimal point in 2.13.

In summary, the lattice method is a written method of multiplying numbers that uses a lattice to break down the process into smaller steps. It is mathematically identical to the more commonly used long multiplication algorithm, but some practitioners find it easier to use.