It is said that paaNini is the greatest grammarian.  It is also said that Lord Siva appeared before him after a long penance, and granted him the vision or knowledge of some rules - suutraas. 

Because of these suutraas, the term ach refers to a set of all vowels (9 in number), hal refers to a set of all consonants (34 in number) and al refers to all letters of the varNa maalaa.  Not only do the terms define, they also imply an enumeration of all the elements of the set that they denote.