any speech sound, such as that represented by t, g, f, or z, that is characterized by an articulation with a closure or narrowing of the vocal tract such that a complete or partial blockage of the flow of air is produced. Consonants are usually classified according to place of articulation (the location of the stricture made in the vocal tract, such as dental, bilabial, or velar), the manner of articulation (the way in which the obstruction of the airflow is accomplished, as in stops, fricatives, approximants, trills, taps, and laterals), and the presence or absence of voicing, nasalization, aspiration, or other phonation. For example, the sound for s is described as a voiceless alveolar fricative; the sound for m is a voiced bilabial nasal stop. Additional classificatory information may indicate whether the airstream powering the production of the consonant is from the lungs (the pulmonary airstream mechanism) or some other airstream mechanism and whether the flow of air is ingressive or egressive. The production of consonants may also involve secondary articulations—that is, articulations additional to the place and manner of articulation defining the primary stricture in the vocal tract.