Because consonants involve a narrowing or "stricture" at an identifiable place in the vocal tract, phoneticians have traditionally classified a consonant in terms of its "place of articulation". The [t] of ten, for instance, requires an airtight seal between the upper rim of the tongue and the upper gum or teeth. Phonetic description of place of articulation, however, concentrates on a section or "slice" through the mid-line of the vocal tract, the mid-sagittal plane as it is know, and in this plane the seal is made between the tip or blade of the tongue and the bony ridge behind the upper front teeth, the alveolar ridge. The sound is therefore described as alveolar.