ɪn ɐ pɑːɹtˈɪʃən təpˈɑːlədʒi ðə kˈɑːmplɪmənt ʌv ˈɛvɹi ˈoʊpən sˈɛt ɪz ˈɔːlsoʊ ˈoʊpən, ænd ðˈɛɹfoːɹ ɐ sˈɛt ɪz ˈoʊpən ɪf ænd ˈoʊnli ɪf ɪɾ ɪz klˈoʊzd.
In a partition topology the complement of every open set is also open, and therefore a set is open if and only if it is closed.