Now let us look at some of the topics that are covered from the point of view of structure and brevity.

1. Definitions or sanjnaa vidhis ( s:�wa ev:eD: )

A number of definitions are used to refer to a set of letters as a group.  Let us look at specific examples to understand how this works.

The entire alphabet is first grouped together in a certain order. This order is as follows :

AEuN:�.  ?l:�k�.  OA:�{�.  O�A:�c:�.   hy:v:rX�.  l:N:�.
W:m:{N:n:m:�.  J:B:W:�.  G:ZD:\:�.   j:b:g:RdS:�.
K:PCYT:c:Xt:v:�.  kp:y:�.  S:\:s:r�.  hl:�..

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(As an aside - These are called the Maaheswara Sutras. The story goes that PaaNini deciphered these aksharas (or letters) from the Damaru (udukkai) sound of Lord Shiva. Apparently that was Lord Shiva's blessings for PaaNini to compose this great work.)

Note certain points in this arrangement.
(i) Every phrase ends in a consonant. These ending consonants are defined as part of a set called 'it' ( Et:�). 'it's are used only for the purpose of continuity in pronunciation, and can be considered to be non-existent for other purposes.

(ii) A word formed with 2 aksharaas - where the first is any of the non-'it' letters and the second is an 'it', refers to all the aksharas which appear between these two in the above sequence. This word is referred to as pratyaahaara (p:�ty:ahar).

A:edrnty:�n: s:h�t:a  is the sutra that defines this.

    For example, ach (Ac:�)- refers to a,e,u,ru,lru,ae,i,o,ou - i.e., all the letters between the first a and the 'it' ch which appears in the 4th phrase. Note that this covers all the vowels. So the vowels are given the name of ach.
    Similarly, hal (hl:�) - refers to the set of all consonants, ak(Ak�)  refers to a,e,u, ek refers to e, u and so on.

(iii) Among the consonants, each phrase has certain common properties. For instance, the group nyam ( W:m:� ) refers to nasal consonants.

    This kind of grouping is very effectively used in specifying rules related to these groups.
 
2. Morphology

    PaaNinian sutras are very rich in morphological rules. These deal with derivation of words,  Sandhi - both internal and external (we'll look at this in some detail), declension of nouns and conjugation of verbs.

A word or pada is derived by adding various kinds of suffixes to the base or stem or root called the anga. The stem could be of two types - verbal stem (D:at:� - dhaatu) or nominal stem (p:�aet:p:edk� - praatipadikam). These stems themselves could be of two types - basic stems and derived stems. Suffixes that can be added to the dhaatu stems and the praatipadika stems are also specified and classified.

One major set of rules regarding the suffixes is about those that are added to nouns for their declension, and another is about those that are added to verbs for conjugation. Declension rules specify the case markers that are taken by the nouns in all three numbers (singular, dual and plural).  Depending on the ending aksharaa of the noun, and its gender, different forms are obtained. Rules are specified for all of these, but using the same set of suffixes called the sup suffixes which are 21 in number.
Similarly, conjugation rules specify the suffixes to be added to verb stems to derive the forms taken by the verbs in all three persons (I person, II person, and III person), and all three numbers, for all possible tenses. Here again the set of suffixes are 9 in number and referred to as the 'thin(g)' ( et:{� ) suffixes.
In forming these words, there is an interplay of characters , i.e., an internal sandhi takes place. The sandhi rules defined, take care of this, as well as, the interplay of characters that occur when two full words are joined together (i.e., external sandhi).

Let us look at some of these  Sandhi rules.
 

3. Gender transformation

    PaaNini accepts the gender of a noun (masculine, feminine, or neuter) as assigned in another subsidiary text called Lingaanushaasana.  However,  rules for the formation of feminine stems have been specified. Here again, all feminine stems are generated using just 3 suffixes - taap, gneep, gneeSh. ( Xap:� {ip:� {i\:� )

4. Another major category of rules is that regarding cases or Kaaraka. ( kark )

5. Yet another category of interest is that of formation of compound words or Samaasa. ( s:m:as: )