Phonology originally means: “The science of speech, sounds and pronunciation, especially as they occur in a particular language.”
Phonology goes together with phonetics. Whereas phonetics is about the physical production and perception of the sounds of speech, phonology describes the way sounds function within a given language or across languages.
Phonology occupied with the distinctive units within a language.
These units are called phonemes. Phonemes are used to divide minimal meaningful sounds. A phoneme is a speech sound that helps us construct meanings. If we replace it with another sound we get a new meaning or no meaning at all.
For example, the phonemes /p/ and /b/ are distinctive units of sound. They can be seen from minimal pairs such as "pin"  and "bin" , which have a completely different meaning, but differ only in one sound.
Other examples to describe the construction of meaning of phonemes:
- Replace the initial consonant (/r/) from rubble  :
- meaningless forms:
- Replace the vowel from rubble :
- nonsense form:
When a phoneme is adapted to the spoken context in which it occurs it does not alter the meaning either for speaker or hearer, these different phonemes are called allophones. Allophones are variants of a phoneme. A sound can be produced physically different as the way our speech organs produce it. That is why the detailed phonetic variants are called allophones.
The phonetic alphabets expressed phonemes as well as allophones. Furthermore phonetic alphabets are used to print these minimal meaningful sounds.
In addition to phonemes, Phonology studies sound alternate and topics such as syllables structure, stress, accent and intonation.
One main point of the Early Modern English Phonology is the “Great Vowel Shift”. The Great Vowel Shift was decisive for important vowel changes and diphthongization. There were more sound and grammar changes at this time.