Kewayon aiki shine saitin duk ƙimar fitarwa mai yuwuwa (wanda kuma ake kira "range values") waɗanda aikin zai iya samarwa don duk ƙimar shigar da za a iya samu. A wasu kalmomi, saitin duk y-values ne ake samu lokacin da aka shigar da ƙimar x daga yankin cikin aikin.Misali, la'akari da aikin f(x) = x^2. . Yankin wannan aikin duk lambobi ne na gaske, amma kewayon shine saitin lambobi na ainihi waɗanda ba mara kyau ba, saboda ba za a iya samun lamba mara kyau a matsayin fitowar aikin ba. Saboda haka, kewayon wannan aikin shine [0, ∞).