A further difference between the two groups of deviations is the fact that the Zihafaat fall only on the Sabab (and there on its second consonant), whist the ‘Ilal alter the Watid in each of the last feet of the two hemistiches as well as in their Sababs. By applying the definite Zihafaat and ‘Ilal rules , and taking the normal form of the feet of each metre as a point of departure, one arrives at the forms actually occurring in the Kasiidas , just as the normal feet are denoted by their 8 mnemonic words, (fa‘uulun , mafaa‘iilun , etc.), which express the normal sequence of their ‘moving’ and ‘quiescent’ consonants, there are also mnemonics denoting the forms which have undergone alteration because of Zihaafaat and ‘Ilal and these indicate the changed sequence of consonants. Thus, for instance , mustafilun , when its Sin is lost, should become mutaf‘ilun. If, however, as in this case, the resulting form is not one linguistically possible in Arabic, then the same sequence of consonants (i.e, the same sequence of ‘longs’ and ‘shorts’ ) is expressed by an equivalent word which is linguistically acceptable, in this case, for instance, by mafaa‘ilun. By contrast with the Usuul forms of the feet, these modifications are known as the Furuu‘ forms of the feet. In the following , the Furuu‘ will be added in brackets, if their form differs from that of the Usuul. Space here does not permit a detailed list of all Zihaafaat and ‘Ilal (cf. for the details of the arabic compendia of the ‘ilm al-‘aruud).
A few examples will be given , however, in order to illustrate the theoretical exposition, and to show how peculiar and complicated this particular part of the system is.
As already stated, the Zihaafaat appear when the Sabab in a line does not posses its full normal form, but shows a change in the second consonant. Then , however, one does not simply speak of a Zihaaf , because this would be ambiguous. In order to describe the Zihaaf accurately, one must state which consonant of a foot is affected, and whether that is a ‘moving’ or a ‘quiescent’ consonant. For example, one can divide the so-called 8 ‘simple Zihaafaat’ into two groups, according to whether a sabab khafiif or a sabab thakiil is attached. Even then, one must denote the eight cases by individual terms. 1) We have a khabn , if the second consonant of a foot is missing, e.g., the sin in مُـ[سْـ]ـتَفْعِلُنْ] [مَفاعِلُنْ = , or the alif in فـ[ـا] عِلُنْ ; we have a tayy , if the 4th consonant is missing , e.g. , the fa of مُسْتَـ[ـفْـ]ـعِلُنْ ; a kabd, in the 5th consonant is concerned, e. g., the nun in فعولن or the ya in مفاعيلن ; and a kaff, when the 7th consonant is missing, e.g., the nun of فاعلاتن
2) In the sabab thakiil, there can either be only the vowel of the second consonant missing (then one speaks of an idmaar , in the case of fatha of mut[a]faa‘ilun [= mustaf‘ilun], and of an ‘asb in the case of the fatha of mufaa‘al[a]tun [= mafaa‘iilun ] or both this consonant and its vowel (then one speaks of a waks , if the ta of mu[ta]faa‘ilun [=mafaa‘ilun] is missing, and of an ‘akl in the case of the la of mufaa[la]tun [= mafaa‘ilun]). Whilst the Zihaafaat always lead to a minus , when compared with the normal Sabab, the ‘Ilal (which change the last feet of the two hemistichs) fall into two groups , according to whether they arise out of an addition (ziyaada) or an omission (naks) . 1) the tathyiil , for example , adds a ‘quiescent’ consonant to the watid madjmou ( thus مستفعلن becomes
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