Arabic phonology is organised in the grammatical tradition by place of articulation, running from the throat outward to the lips, and the classification is old: it is set out in the eighth century, in the work associated with al-Khalil ibn Ahmad and in the account of his pupil Sibawayh, and it is still the framework in which the recitation manuals are written. Six consonants are assigned to the throat: the glottal stop and the plain h at the outermost point, ayn and the pharyngeal ha in the middle, and ghayn and kha at the point nearest the mouth.
The grouping is not merely descriptive. It carries rules. Arabic word roots do not freely combine consonants from the same articulatory group, so a root containing ayn is unlikely also to contain ha, and lexicographers used the constraint to organise dictionaries. In recitation the throat letters govern one of the best known rules of tajwid, under which a nun with no vowel is pronounced clearly rather than assimilated when a throat letter follows it. That rule is taught early and by name.
The same six letters are the ones that European learners find hardest and that European transliteration handles worst. Ayn and the glottal stop are frequently dropped altogether in roman spellings of Arabic names, ghayn becomes gh and kha becomes kh by convention, and the two h sounds are usually written with the same letter. The atlas follows the common convention of transliterating without diacritics, and distinguishes the pairs by name where the distinction matters rather than by marks a reader cannot see.