The obligatoriness of English bisentential too varies with the prominence of the focus on contrasting constituents. This fact leads to a possible explanation for its obligatoriness: too is obligatory where an overt element is needed to emphasize what is important about the content of a two-clause text—i.e., when what is important is that the same thing is predicated about two contrasting items. Too is just the element to do this, because of what it conventionally implicates (that what is predicated by the speaker about the contrasting item in one clause is also predicated about the contrasting item in the other) and because of its hypothesized discourse function (to emphasize the similarity between contrasting constituents).