For convenience in keeping the list of differentia reasonably small, to avoid unwieldiness of definition, care must be exercised in choosing the class. When a class which itself contains other possible classes is chosen, a long list of differentia will be necessary. It is well, therefore, to choose a relatively small class to begin with. For example, if I put the piano into the large class of _musical instruments_, I shall then be under the necessity of amassing sufficient differentia to set it apart from wind instruments whether of brass or wood, from instruments of percussion, and from other stringed instruments that do not use metal strings. If I restrict the class to _stringed instruments_, I thereby exclude the differentia of both wind instruments and instruments of percussion. If I further restrict the class, at the beginning, to _instruments with metal strings_, I need then to employ only such differentia as will set it off, perhaps, from instruments that do not have a sounding board for their metal strings. Such restriction of the class is advisable chiefly for purposes of economy of effort in discovering the differentia, and is usually accomplished, in expression, by preceding the class name with a limiting adjective or by using a limiting phrase. This adjective or this phrase is likely to be the expression of differentia among smaller classes, the differentia among individual members being stated more at length later in the definition.
The process of definition will be complete, then, when the subject of definition has been assigned to a class, which for convenience should be relatively small, and the qualities that distinguish the subject from other members of the class have been found.
Two main classes of definition exist: first, the rigidly logical, scientific kind such as is found in dictionaries, textbooks, and other such writings which are not concerned with emotional values; and second, the less rigid, more expanded, more informal kind which aims to please as well as to instruct, and which is found in essays and all forms of writing with a strong human appeal. The two kinds are alike in the presence of both genus and differentia; they differ chiefly in the presence, in the less formal, of the qualities of pleasingness and stimulation as opposed to the quality, in the formal, of scientific impersonality, cold intellectuality. For example, the Standard Dictionary defines a _correspondent_ as "one who communicates by means of letters; specifically one who sends regular communications from a distant place to a newspaper or a business house." The author of the volume entitled _Famous War Correspondents_[14] defines, with much the same fundamental ideas, if not indeed exactly the same, a _war correspondent_ as follows: