Words such as "adept," "about," "occur" and "along" start with an unstressed mid-central neutral vowel sound, commonly referred to as the schwa. Some, but not all, words that begin with the letter "a" or "o" as the initial syllables and are followed by stressed second syllables beginning with consonants are pronounced with a schwa sound.
The schwa sound, pronounced as "uh," is represented in regular orthography by any of the six vowels in the English language. Occasionally, as with the second syllable in the word "rhythm," the schwa sound is present but not represented due to spelling shortening of the word over time. In the International Phonetic Alphabet, used to transcribe specific phonemes, it is represented by an inverted and rotated lowercase letter "e." The term schwa comes from the Hebrew word "shva" and was first used by German linguists in the early 19th century.
Also referred to as the natural, toneless or reduced vowel, it is the most commonly used sound in the English language. Its description as a mid-central vowel notes the position of the tongue during utterance on two criteria: its height and backness. The latter refers to whether it is closer to the teeth or rear of the mouth, while the former indicates proximity to the palate or low with the mouth open.