I have seen some geminate consonants written both as, for example /mm/ and /m:/. Is there any difference between the two (assuming there is no syllable boundary), or is it just a question of notational style? Originally I had only ever seen the length marker used on vowels, I remember being quite flummoxed the first time I saw it used with a consonant! A thought has just occurred, is the use of : restricted to sonorants? When I think about it, I don't recall having seen it used with a stop.