Ancient Indians used the joints of their fingers to count, a practice still seen today. Each finger has 3 joints, and the thumb is used to count them. How many can you count on one hand? How does this relate to the ancient base-12 counting systems? [Page No. 43]

One hand has 4 fingers, excluding the thumb, and each finger has 3 joints.
So, the total number of joints is 4 x 3 = 12.
This number, 12, forms the base of the duodecimal system. In ancient times, people used the thumb as a pointer to count these joints and reach up to 12 on one hand. With both hands, one hand counted up to 12 while the other kept track of the number of groups, allowing counting up to 144, called a gross.
This is one reason why the base 12 system was commonly used in history, such as in 12 months of a year, 12 inches in a foot, and 12 hours on a clock.