A sonority hierarchy or sonority scale is a ranking of speech sounds (or phones) by amplitude. For example, if one says the vowel [a], he or she will produce a much louder sound than if one says the stop [t]. 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-).