The purpose of writing is to represent speech. In writing, a unit of print represents a unit of the speech stream. Writing systems differ, however, in how they define ‘unit of the speech stream’ for the purpose of writing. In a logography such as Chinese, the unit of speech represented by a character is the morpheme. A syllabary, such as the Kana scripts of Japanese, represents each syllable with a single character. A large majority of the world's languages today use an alphabetic system, in which each character (or combination of characters) represents a single phoneme.

Alphabetic systems also vary in the consistency of the mapping between print and sound. In ‘shallow’ orthographies (such as Spanish, Italian, or, in the present case, Korean), there is close correspondence between written characters and the phonemes they represent. In a ‘deep’ orthography such as English, this correspondence is compromised. For example, the letter c may be pronounced either as /k/, as in cat, or /s/, as in circle. A number of studies have examined the degree to which a reader's knowledge of the sound system affects the processing of printed words. Because of the many inconsistencies found in English in the relation between spelling and pronunciation, it is perhaps not surprising that it has been the focus of so much of the research in this area.

Much of this research has investigated issues concerned with word regularity.