English Phonology and Phonetics— Phoneme

"The Phoneme

The "p" sound in the data is pronounced in three different ways.
This difference in pronunciation is not random, but predictable, depending upon where the "p" sound occurs in the word:
. . .the different pronunciations occur are unique, and do not overlap.  . . .
Since these different pronunciations never appear at the same place in words, they are said to be in complementary distribution.
 . . .


 . . .
Take your hand and place it in front of your mouth. Now say pat.

Did you feel the air against your hand? Now say spat.

When you say pat, you should feel more air against your hand than when you say spat. That extra puff of air is known as aspiration, and is represented in the transcription by the superscript "h".

Now, often, when one says tap, tip or top, there is no release of the "p" sound, and no air is present when the sound is articulated. This is known as an unreleased sound, represented by the superscript "┐" next to the sound.
. . .

A phoneme is a mental representation of a sound that has predictable variants. Each of the variants of that sound is called an allophone."
http://www.ic.arizona.edu/~lsp/Phonology/Phonology1c.html
allophone: [ ]
phoneme: / /

"

Overlapping Distribution
Sometimes, sounds do appear in at least some of the same contexts. When this happens, the sounds are in overlapping distribution.

For example, observe the following data:


,,,
...
if these sounds are in overlapping distribution, they must be variants of separate phonemes.
"

http://www.ic.arizona.edu/~lsp/Phonology/Phonology1e.html


Summary

If sounds are in complementary distribution, they cannot appear in the same contexts, and are allophones of the same phoneme.
If sounds are in overlapping distribution, they are allophones of different phonemes.