They also classify “particles” according to their meanings into interrogative particles, negative particles, particles of specification, and so forth. According to their structure, they classify them—as they do verbs and nouns—into one-, two-, three-, four-, and five-letter forms. Here, the term harf has been transferred from ordinary language into grammatical usage by restricting its meaning. In ordinary language, however, harf encompasses nouns, particles, and verbs. The alphabetic letter-names are also called hurūf, although they are nouns, as with the letters mentioned at the beginnings of surahs: what each name denotes is a letter forming part of a word.

Letters are also classified in another way, into guttural and labial letters. Those mentioned at the beginnings of surahs in the Quran constitute half the alphabet and include the nobler half of each class: half the guttural letters, the labial letters, the emphatic letters, the musmata (non-fluid) letters, and other classes of letters.

The original meaning of harf in the language is a boundary or edge, as in the expressions “the edges of a loaf” and “the edge of a mountain.” Al-Jawharī said:

The harf of anything is its extremity, edge, or boundary. Hence the expression “the harf of a mountain,” meaning its sharp summit, and the words of the Most High:

“And among people are those who worship Allah on an edge,”

through His words:

“and the Hereafter.”

Someone standing at the edge of something is not firmly established. Thus, whoever worships Allah in prosperity but not in adversity worships Him on an edge: at times he displays this worship, and at others he turns