View source for Unordered pair

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''doubleton''

A set containing two elements, denoted $\{x,y\}$.  One has $z \in \{x,y\} \Leftrightarrow z = x \vee z = y$, and so $\{x,y\} = \{y,x\}$.  Compare with an [[ordered pair]].

====References====
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<TR><TD valign="top">[1]</TD> <TD valign="top">  P. R. Halmos, ''Naive Set Theory'', Springer (1960) ISBN 0-387-90092-6</TD></TR>
</table>