i (/aɪ/), always lower case, can be used in algebraic notation to represent language level, following Krashen, in the context of his comprehensible input hypothesis:
- i-1 is language a level easier, and well understood in the given context. Most extensive reading should be conducted at this level.
- i+0 or simply i is the language level.
- i+1 is language a level harder, not well understood in the given context.