A phoneme is the smallest unit of sound in a language. It is the smallest contrastive unit that can change the meaning of a word. Phonemes can be categorized according to their features. These categories, or phoneme classes, are based on the features of the sound such as string of articulatory features, position of articulation, manner of articulation, voicing, and place of articulation. For example, a voiceless stop consonant is a phoneme class. A stop consonant is a type of sound which is produced by obstructing airflow in the vocal tract. Voiceless means there is no vocal cord vibration when producing the sound. All voiceless stop consonants would have the same set of articulatory features and be produced in the same way. The four major phoneme classes are consonants, vowels, liquids, and semivowels. These classes are further divided into sub-classes based on their articulatory features. Consonants are phonemes produced by blocking airflow in the vocal tract, which results in a noise or burst of air. Depending on how much airflow is blocked and how hard or soft the sound is, consonants can be divided into sub-classes such as stops, affricates, fricatives, and nasals. Vowels are voiced sounds produced without obstruction of airflow in the vocal tract.The main feature of a vowel is the tongue’sheight and movement. Vowels can be further divided into monophthongs, diphthongs, triphthongs, closed vowels, and open vowels. Liquids are characterized by the tongue's relaxed shape and fast time of articulation. They can be divided into rhotics and laterals. Semivowels are voiced sounds produced by slightly obstructing airflow in the vocal tract. This produces a glide between two other sounds. Semivowels can be either palato-alveolar or glottal. In summary, phonemes can be sorted into four phoneme classes, with each phoneme class having its own sub-classes based on their articulatory features. These classes are used to create the phonology of a language or dialect.