By agreement we mean a method of expressing a syntactical relationship, which consists in making the subordinate word take a form similar to that of the word to which it is subordinate. In Modern English this can refer only to the category of number: a subordinate word agrees in number with its head word if it has different, number forms at all. This is practically found in two words only, the pronouns this and that, which agree in number with their head word. Since no other word, to whatever part of speech it may belong, agrees in number with its head word, these two pronouns stand quite apart in the Modern English syntactical system.