Lattice Method of Multiplication

Lattice multiplication, a variations of today's
standard long multiplication were introduced into
Europe by Fibonacci (whose correct name is
Leonardo of Pisa). It requires the preparation of a
lattice (a grid drawn on paper) which guides the
calculation and separates all the multiplications from
the additions.

During the multiplication phase, the lattice is filled in
with two-digit products of the corresponding digits
labeling each row and column: the tens digit goes in the
top-left corner.
During the addition phase, the lattice is summed on the
diagonals.

Finally, if a carry phase is necessary, the answer as
shown along the left and bottom sides of the lattice is
converted to normal form by carrying ten's digits as in
long addition or multiplication.