The advantage of this system is evident.  It enables the student, who has once grasped the original meaning of a root, to form scores of words himself, and in his readings, to understand hundreds, nay thousands, of words, without recourse to the Dictionary, as soon as he has learned to distinguish their radical letters from the letters of increase, and recognises in them a familiar root.  We cannot wonder, therefore, that the inventor of Arabic Prosody readily availed himself of the same plan for his own ends.  The Taf’il, as it is here called, that is, the representation of the metrical feet by current derivatives of fa’l, has in this case, of course, nothing to do with the etymological meaning of those typical forms.  But it proves none the less useful in another direction:  in simply naming a particular foot it shows at the same time its prosodical measure and character, as will now be explained in detail.

We have seen supra p. 236 that the word Akamu consists of a short syllable followed by two long ones (U — -), and consequently forms a foot, which the classics would call Bacchius.  In Latin there is no connection between this name and the metrical value of the foot:  we must learn both by heart.  But if we are told that its Taf’il in Arabic is Fa’ulun, we understand at once that it is composed of the Watad majmu’ fa’u (U -) and the Sabab khafif lun (-), and as the Watad contains three, the Sabab two letters, it forms a quinqueliteral foot or Juz khamasi.

In combining into feet, the Watad has the precedence over the Sabab and the Fasilah, and again the Watad majmu’ over the Watad mafruk.  Hence the Prosodists distinguish between Ajza asliyah or primary feet (from Asl, root), in which this precedence is observed, and Ajza far’iyah or secondary feet (from Far’= branch), in which it is reversed.  The former are four in number:- -

1.  Fa’u.lun, consisting,as we have just seen, of a Watad majmu’ followed by a Sabab khafif = the Latin Bacchius (U — -).

2.  Mafa.’i.lun, i.e.  Watad majmu’ followed by two Sabab khafif = the Latin Epitritus primus (U — — -).

3.  Mufa.’alatun, i.e.  Watad majmu’ followed by Fasilah = the Latin Iambus followed by Anapaest (U — UU -).

4.  Fa’i.la.tun, i.e.  Watad mafruk followed by two Sabab khafif = the Latin Epitritus secundus (-U- -).

The number of the secondary feet increases to six, for as Nos. 2 and 4 contain two Sabab, they “branch out” into two derived feet each, according to both Sabab or only one changing place with regard to the Watad.  They are: 

5.  Fa.’ilun, i.e.  Sabab khafif followed by Watad majmu’= the Latin Creticus (-U-).  The primary Fa’u.lun becomes by transposition Lun.fa’u.  To bring this into conformity with a current derivative of fa’l, the initial Sabab must be made to contain the first letter of the root, and the Watad the two remaining ones in their proper order.  Fa is therefore substituted for lun, and ’ilun for fa’u, forming together the above Fa.’ilun.  By similar substitutions, which it would be tedious to specify in each separate case, Mafa.’i.lun becomes: