Triangular colon

The triangular colon or IPA long-vowel mark is a symbol that in small fonts looks like a colon and indicates a vowel that is longer than others.

Examples:

fleece /fliːs/

fleece /fliːs/

In general the triangular colon can be replaced by the normal colon.

Standard use of the triangular colon

 * IPA phoneme /ɑː/, as in palm, father, bath, start
 * Phoneme /ɑː/ in General American, as in palm, father, start, lot
 * IPA phoneme /iː/, as in fleece, happy
 * IPA phoneme /ɔː/, as in thought, cloth, north, force
 * IPA phoneme /uː/, as in goose
 * IPA phoneme /ɜː/, as in nurse

Non-standard use of the triangular colon
The Concise Oxford Dictionary and Oxford Dictionaries Online use /əː/ instead of /ɜː/; This change has been referred to as having "the advantage of reducing the total number of unfamiliar symbols to be assimilated", and also "as things are, it doesn't now seem worthwhile changing what we have".

Also "the substitution of /ɛː/ for /eə/, is one with which one can have a great deal of sympathy". However the same reference says "it is surely best to leave EPD14 just as it is".

Redundancy
Since there are no phonemes /ɑ, i, ɔ, u/ or /ɜ/, the long-vowel mark can be suppressed with no loss of information. This is common practice in American dictionaries. On the other hand, the opposite can be done, and use /a, a:, i, i: o, o:, u/ and /u:/ instead of /a, ɑ, ɪ, i, ɒ, ɔ, ʊ/ and /u:/ (see "Reduced symbol set" in the table below):