The Oper language grading scheme was especially compiled by D.H. Howe as a guide to the preparation of language teaching material for school pupils and adults learning English as a second or foreign language. The scheme provides lists of words and language structures subdivided into there grades of difficulty and meant to be used in conjunction with each other.
The items were chosen according to two main principles: first, that they are likely to have been learnt or at least encountered before the stage indicated, second, that they are frequently occurring and useful, necessary to express a wide range of ideas, and difficult to replace with simpler words or constructions. Us of the scheme is intended to eliminate unnecessary difficulties of language which would otherwise hinder understanding and enjoyment.