Lattice multiplication stems from the ninth century Persian mathematician Muhammad Al-Khwarizmi. The Italian mathematician Fibonacci brought the practice to Europe sometime around the 1200s in Europe. The practice got its name from the grid you fill in to multiply your numbers. It resembles a latticework frame that forms a crisscross pattern.
Regardless of decimal points, the Lattice box is set up the same way. You start by drawing a grid that has as many rows as the multiplicand and as many columns as the multiplier. Next, draw a diagonal line through each box from upper right corner to lower left corner. Continue the line slightly past the grid. Write the multiplicands across the top and the multiplier down the right side, lining up the digits with the boxes.
Start by multiplying the digits at the head of each row and column. Fill in each square of the grid with the product of the digits above and to its right, recording the products so that the tens are in the upper (diagonal) half of the square and the ones are in the lower half. If the number doesn't have a tens digit, put a zero in its place. Now add the numbers in the grid along the diagonals, starting from the lower-right corner, carry any tens into the top of the next diagonal. For your answer, read the numbers starting down the left of the grid and continuing across the bottom
Multiplying the Lattice method with decimals is simple once you know how to use the Lattice method with whole numbers. Start the process as you normally would, but draw lines from the decimal points down from the number on top and to the left from the number on the left. When the lines meet, follow the diagonal to the left or bottom of the grid. The point where this diagonal emerges from the grid is the position of the decimal point in the answer. Make your Lattice box as you normally would; simply place your decimals along the dividing lines of the rows and columns. Continue multiplying and adding the numbers as you normally would. As an alternative, when you add your diagonal numbers, count up how many decimal places were in each number you multiplied. Then, add the number of decimal places. Starting from the end of the result, move your decimal point that many decimal places.