A tatami mat consists of a thin layer of tightly woven rushes, on top of a coarser mat of straw bound with cords. The upper mat is sewn to the lower one with twine. The mat is firm but not hard.

In the United States, building dimensions often are dictated by the size of a standard sheet of plywood: 4 feet by 8 feet.

The sequence in Additional Challenge 1b is a Fibonacci sequence. Fibonacci was the nickname for the Italian mathematician Leonardo of Pisa. In his book Liber abaci, published in 1202, Fibonacci described the following problem:

How many pairs of rabbits can be produced from a single pair in a year, if every month each pair bears a new pair which, from the second month on, also becomes productive?

The resulting sequence of the pairs at the end of each month, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55,... is known as the Fibonacci sequence.