It takes two to tango is a common idiomatic expression which suggests something in which more than one person or other entity are paired in an inextricably-related and active manner, occasionally with negative connotations. The tango is a dance which requires two partners moving in relation to each other, sometimes in tandem, sometimes in opposition. The meaning of this expression has been extended to include any situation in which the two partners are by definition understood to be essential—as in, a marriage with only one partner ceases to be a marriage.