Triangular colon
From Hull AWE
ː
The triangular colon is a special mark developed by the IPA to indicate that a vowel is long. Note that it properly consists of two isosceles triangles with their apexes facing each other, rather than the two points of the usual colon of the English alphabet. However, this only applies to careful typing, or typesetting: even pedantic phoneticians normally use two dots when transcribing long vowels by hand.
