Defining regularity in the English language is not a straightforward matter because of the complex relations between sound and spelling. Predictable patterns are those spellings that can be readily predicted from pronunciation. Rare and unpredictable spellings are found only in a small set of words--probably 10% or less of the vocabulary in English. However, the boundary between rare and frequent unpredictable patterns is unclear. Since rarity has no absolute values, such distinctions in instruction must be made on other grounds such as the usefulness of the words. A three-way categorization, which parallels a linguistic classification based on sound-to-spelling correspondences and which also includes eight categories relevant to learning to spell, seems most suitable for the design and development of spelling instruction. These three categories are based on the type of learner behavior appropriate for different words and rules. The first class (predictable spellings) covers words that students should be able to spell simply by listening to the sounds and applying the rules that have been taught. The second class (unpredictable and rare spellings) covers words that must be memorized by sight, and the third class (unpredictable but common spellings) covers words in which the sounds should help students know how the words may be spelled, but students must learn the words by memory or must check in a dictionary in order to determine the exact spelling. (HOD)