Subject approach: In academic, special, technical and research libraries, and to a large extent even in public libraries, you will find that the majority of readers approach documents on the basis of the subject. This subject approach by readers has increased due to the growth in science and technology and also to a large extent in social sciences where the author and the title of a document are important but not adequate. Due to the enormous output of documents in these fields, it is often difficult to recall a specific title or author correctly except in the case of classics.
Therefore, one finds that in well organised libraries documents are arranged on the basis of subject matter. This arrangement helps to bring documents embodying the same specific subject together and those on related subjects in their close proximity on the shelves. This would result, as Ranganathan puts it, into an APUPA arrangement that will give reader the greatest satisfaction at the moment in full conformity to all the Five Laws of Library Science. In this arrangement, the focal point of one's main interest is the UMBRAL REGION. This is followed on, either side by the PENUMBRAL REGION represented by subjects on either side of the UMBRAL region having successfully a decreasing bearing on the umbral region. The penumbral region will ultimately thin into ALIEN REGIONS. This is helpful and convenient especially on open access libraries.
Readers can directly go to the shelves and browse through the library collection. Generally, the objective of libraries is to have an APUPA ARRANGEMENT EVERYWHERE. Subject arrangement, hence, helps bring together on the shelves documents on one and the same subject followed by those of related subjects. Suppose you are looking for a specific document on say, physics, you can find, where subject arrangement exists, documents not only on physics, but also those related to it in close proximity, i.e., chemistry. It may so happen that if a specific title wanted by you is not available, you can find another title on the subject that may very well meet your requirement. Some scholars may need everything available on their topic. You will notice that near the books on chemistry, you will also find books on related subjects like mathematics and astronomy on one side and chemistry and biology on the other.