Lenition is a consonant mutation that weakens the sound a consonant makes within a word. This change can take place anywhere within the word, depending on the nature of the language or dialect concerned. There are four major types of lenition: spirantization, the opening of fricatives, debuccalization, and deletion. These changes are either made synchronically, as an active change within the modern language, or diachronically, as a fossilized change with the language's development. The purpose of this mutation is to reduce the amount of air flow stoppages caused by consonants during speech.
Spirantization is the process of making a sound more fricative, which involves forcing air through a narrow passage that is created within the mouth by the lips or by the tongue's interaction with the teeth or palate. By forcing the air through a plosive stop, the speaker is improving the flow of sounds and is therefore creating a fricative. An alternative to this is to make the consonant that disrupts the flow of air into a glottal consonant in a process called debuccalization. In some cases the consonant can be deleted altogether in speech, but may still remain in the word's written form.
Changing a consonant's form mutates its level of sonority. To be more sonorous means to be more like a vowel. The effect of making consonants more vowel-like is that they reduce the amount of breaks within a sentence. A consonant that has been through the process of lenition is called a lenited consonant. The opposite of making a consonant more sonorous is called fortition.
Lenited consonants are affected by two sets of adjacent letters around them. Changes may depend on the vowels that immediately surround the consonant, and the strength of other consonants beyond the surrounding vowels have an effect too. If there are too many strong consonants, and therefore too many plosive stops, the middle consonant will lenite. The strength of a consonant in linguistics is determined by its resistance to airflow it causes during speech.
Synchronic lenition is a more active form of consonant weakening. These changes are an active part of a language's grammar and word morphology. A basic example is the addition of "n" to "a" when placed before a vowel in English. This makes "a horse" and "an apple." The same process happens in Hungarian with "the" to make a gally or "the twig" and az esku or "the oath." As seen with these examples, some lenited consonants are affected by sounds in adjacent words as well as sounds within the word itself.
In Irish and other Celtic languages, the relationship between syntax and consonant sound are regulated. For example, in Irish, the lenited consonant is always shown by having the letter "h" placed after it. This can take place at any point within the word if certain rules are met. In Welsh, the first letter of a word like cath meaning "cat," can change to gath under certain syntactical circumstances.
Diachronic lenition is a change in the sonority of a consonant that took place in a language's past. Examples of this phenomenon include the change from Old English to Modern English and the change from Latin to Spanish. In these lenitions, the sound structure of the language goes through a profound change, but the basic words and grammar remain the same.
In German, the process of lenition occurred during the language's development from Proto-Indo-European to Proto-Germanic. This change is called "Grim's Law" and has three elements: voiced frigatives were made out of voiced aspirated stops, voiceless stops out of voiced stops, and voiceless frigatives out of voiceless stops. When a number of sound elements shift along a sound scale at the same time, the process is called a chain shift.