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 ( 90, 27 ) or the natural number 7 to see how it may be defined as a set.
(90, 27, 7) = ( ( 90, 27 ), 7 )