To generate any other string, it needs to start with the production SaSb and then the non-terminal S in the RHS can be replaced either by ab (in which we get the string aabb) or the same production SaSb can be used one or more times. Every time it adds an 'a' to the left and a 'b' to the right of S, thus giving the sentential form . When the non-terminal is replaced by ab (which is then only possibility for generating a terminal string) we get a terminal string of the form .

There is no general rule for finding a grammar for a given language. For many languages we can devise grammars and there are many languages for which we cannot find any grammar.