A sonority hierarchy or sonority scale is a hierarchical ranking of speech sounds (or phones). Sonority is loosely defined as the loudness of speech sounds relative to other sounds of the same pitch, length and stress, therefore sonority is often related to rankings for phones to their amplitude. For example, pronouncing the vowel [a] will produce a louder sound than the stop [t], so [a] would rank higher in the hierarchy. However, grounding sonority in amplitude is not universally accepted. Instead, many researchers refer to sonority as the resonance of speech sounds. This relates to the degree to which production of phones results in vibrations of air particles. Thus, sounds that are described as more sonorous are less subject to masking by ambient noises. Sonority hierarchies are especially important when analyzing syllable structure; rules about what segments may appear in onsets or codas together, such as SSP, are formulated in terms of the difference of their sonority values. Some languages also have assimilation rules based on sonority hierarchy, for example, the Finnish potential mood, in which a less sonorous segment changes to copy a more sonorous adjacent segment (e.g. -tne- → -nne-). Sonority hierarchy Sonority hierarchies vary somewhat in which sounds are grouped together. The one below is fairly typical: Sound types are the most sonorous on the left side of the scale, and become progressively less sonorous towards the right (e.g., fricatives are less sonorous than nasals). The labels on the left refer to distinctive features, and categories of sounds can be grouped together according to whether they share a feature. For instance, as shown in the sonority hierarchy above, vowels are considered [+syllabic], whereas all consonants (including stops, affricates, fricatives, etc.) are considered [−syllabic]. All sound categories falling under [+sonorant] are sonorants, whereas those falling under [−sonorant] are obstruents. In this way, any contiguous set of sound types may be grouped together on the basis of no more than two features (for instance, glides, liquids, and nasals are [−syllabic, +sonorant]). Sonority scale In English, the sonority scale, from highest to lowest, is the following: In simpler terms, the scale has members of the same group hold the same sonority from the greatest to the smallest presence of vibrations in the vocal folds. Vowels have the most vibrations, but consonants are characterized as such in part by the lack of vibrations or a break in vibrations. The top of