An n-tuple (x1,...,xn) is an ordered list of n sets. It can be constructed in several ways. A popular definition is the original set x1 for n=1 and the ordered pair ((x1,...,xn-1),xn) otherwise.
Klick on the ordered pair ( 99, 81 ) or the natural number 2 to see how it may be defined as a set.
(99, 81, 2) = ( ( 99, 81 ), 2 )