Xisaab ahaan, waxay ahaanaysaa urur uun urur kooban haddii laga helo xiriirka Midmid ku dhexjirta iyo Urur kale leh qaabka {1, 2, ..., n} kolka n waana Tiro tirsiimo. Tusaale ahaan, Ururta { 2 , 4 , 6 , 8 , 10 } {\displaystyle \{2,4,6,8,10\}\,\!} waana urur kooban tirada shanta qaybood. Waxaa loo ogolyahay qiime ahaan in ay ka koobnaato n = 0 Sidaa darteed maxaa yeelaay urur madhan waana ururu kooban. Waxaa la yiraah Urur kastoo S, waa koobantahay haddii la helo Midmid f : S → { 1 , … , n } {\displaystyle f\colon S\rightarrow \{1,\ldots ,n\}} marka n Tiro tirsiimo Ogaal guud. Haddii urur uun kooban, waxaa loo qarayaa susun sidaan: S = { x 1 , x 2 , … , x n } . {\displaystyle S=\{x_{1},x_{2},\ldots ,x_{n}\}.} Tiro susun Patrick Suppes, Axiomatic Set Theory, D. Van Nostrand Company, Inc., 1960 Maqaalkan waa la bilaabay.