Phonology distinguishes between two types of neutralization: passive and active. Neutralization is a phonological process whereby a phonetic contrast is eliminated. For example, English has a high front unrounded tense vowel in "mate" and a high front unrounded lax vowel in "met". However, in word-final position, this contrast in eliminated because only the high front unrounded tense vowel occurs. This is exemplified in the words "say" and "obey".
Many languages have a voiceless and a voiced alveolar plosive. In English the words "two" and "do" are distinguished by voiceless and voiced varieties of this alveolar plosive. After a word-initial voiceless alveolar fricative, though, only the voiceless alveolar plosive occurs. This can be seen in the words "stay" and "stone". Here the distinction between the voiced and voiceless alveolar plosives is neutralized. This neutralization is passive because it is natural that only a voiceless alveolar plosive should occur after a voiceless alveolar fricative. This can be explained as the result of a voicing assimilation.
In German, the voiceless and voiced alveolar plosives also occur but in word-final position the contrast is neutralized. This can be seen in the words "Rad" (advice) and "Rat" (wheel). In both cases, the final consonant is voiceless. Note that though "Rad" is spelt with a "d" it is pronounced as a voiceless plosive. This type of neutralization is classified as active neutralization because it is phonetically plausible to maintain a contrast and many languages do so such as English. For example, English has the contrast in the word pair "rode" and "wrote". If a language neutralizes a contrast in an environment in which the neutralization is not phonetically conditioned, this is called active neutralization.
Neutralization is a common phonological process in which a phonetic contrast is eliminated. Two types of neutralization, active and passive, are common in the languages of the world.