Individual sounds serve as the basic units that comprise speech. Sometimes, a person may have difficulties acquiring particular sounds which may impact his or her intelligibility, or the degree to which that person is understood by others. Errors of speech sounds can be described as substitutions, omissions, distortions, and additions. Speech sound disorders are typically divided into articulation disorders (difficulty executing the muscular coordinations necessary to produce a specific sound or sounds) and phonological disorders (lack of understanding of the rules that govern the production and combination of speech sounds). Speech therapy has been demonstrated as an effective tool in improving or correcting these disorders.