Manner of articulation  In articulatory phonetics, the manner of articulation is the configuration and interaction of the articulators (speech organs such as the tongue, lips, and palate) when making a speech sound. One parameter of manner is "stricture," that is, how closely the speech organs approach one another. Others include those involved in the r-like sounds (taps and trills), and the sibilancy of fricatives. The concept of manner is mainly used in the discussion of consonants, although the movement of the articulators will also greatly alter the resonant properties of the vocal tract, thereby changing the formant structure of speech sounds that is crucial for the identification of vowels. For consonants, the place of articulation and the degree of phonation of voicing are considered separately from manner, as being independent parameters. Homorganic consonants, which have the same place of articulation, may have different manners of articulation. Often nasality and laterality are included in manner, but some phoneticians, such as Peter Ladefoged, consider them to be independent. Manners of articulation with substantial obstruction of the airflow (stops, fricatives, affricates) are called obstruents. These are prototypically voiceless, but voiced obstruents are extremely common as well. Manners without such obstruction (nasals, liquids, approximants, and also vowels) are called sonorants because they are nearly always voiced. Voiceless sonorants are uncommon, but are found in Welsh and Classical Greek (the spelling "rh"), in Standard Tibetan (the "lh" of Lhasa), and the "wh" in those dialects of English that distinguish "which" from "witch". Sonorants may also be called resonants, and some linguists prefer that term, restricting the word 'sonorant' to non-vocoid resonants (that is, nasals and liquids, but not vowels or semi-vowels). Another common distinction is between occlusives (stops, nasals and affricates) and continuants (all else). From greatest to least stricture, speech sounds may be classified along a cline as stop consonants (with "occlusion", or blocked airflow), fricative consonants (with partially blocked and therefore strongly turbulent airflow), approximants (with only slight turbulence), and vowels (with full unimpeded airflow). Affricates often behave as if they were intermediate between stops and fricatives, but phonetically they are sequences of a stop and fricative.