The theme upon which and around which a fugue is constructed is commonly identified as the Subject. A melody to be suitable for this purpose will generally exhibit three characteristics:
1.it is usually brief, but embodies a complete musical idea;
2.it is easily recognizable as it weaves through the contrapuntal texture, and
3.it lends itself well to contrapuntal and harmonic manipulation and exploitation.
From the craftsman's point of view the last attribute is the most important. Bach, as also did other composers, employed subjects that range in length from a few notes...