For the concept of the supplement . . . harbors within itself two significations whose cohabitation is as strange as it is necessary. The supplement adds itself, it is a surplus, a plenitude enriching another plenitude, the fullest measure of presence. It cumulates and accumulates presence. . . .
But the supplement supplements. It adds only to replace. It intervenes or insinuates itself in-the-place-of; if it fills, it is as if one fills a void. If it represents and makes an image, it is by the anterior default of a presence. Compensatory [suppléant] and vicarious, the supplement is adjunct, a subaltern instance which takes-(the)-place [tient-lieu]. As substitute, it is not simply added to the positivity of a presence, it produces no relief, its place is assigned in the structure by the mark of an emptiness.
This second signification of the supplement cannot be separated from the first. . . . Each of the two significations is [in turn] effected or becomes discreetly vague in the presence of the other. But their common function is shown in this: whether it adds or substitutes itself, the supplement is exterior, outside of the positivity to which it is super-added, alien to that which, in order to be replaced by it, must be other that it.
Jacques Derrida, Of Grammatology, trans. Gayatri C. Spivak (Baltimore: John Hopkins University Press, 1976), pp. 144-145.