Page "Allophone" Paragraph 22

Since phonemes are abstractions of speech sounds, not the sounds themselves, they have no direct phonetic transcription.

When they are realized without much allophonic variation, a simple ( i. e. ' broad ') transcription is used.

However, when there are complementary allophones of a phoneme, so that the allophony is significant, things become more complicated.

Often, if only one of the allophones is simple to transcribe, in the sense of not requiring diacritics, then that representation is chosen for the phoneme.