The adjective itself refers to sounds from the throat. In language studies this refers to letters that are throaty in sound and are very common particularly in the Semitic family of languages. In biblical Hebrew, the letters, 'aleph א , he ה, heth ח, 'ayin ע, and resh ר, are treated as gutturals, and particular rules apply to them with regards to the vowel points they may carry and to the definite article attached to them.
For more, see:
Gesenius' Hebrew Grammar: select from the bookmarks the follow §22-23, or page 77, for "Peculiarities of the Gutturals."
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