The prefix 'il-' is a variant of 'in-' meaning 'not'. Illegal means not legal. Similarly, illogical means not logical, and illiterate means not literate. The 'il-' form is used before words starting with 'l'. 'Between' is 'inter-', 'within' is 'intra-', and 'alone' relates to 'sol-' or 'uni-'.