› . ▁ ▁In ▁Bur k ina ▁F aso , ▁the ▁Di ou la ▁alphabet ▁is ▁made ▁up ▁of ▁ 2 8 ▁letters ▁each ▁representing ▁a ▁single ▁ph on eme . ▁In ▁the ▁orth ography , ▁long ▁v ow els ▁are ▁represented ▁by ▁double d ▁letters ; ▁thus , ▁/ e / ▁is ▁written ▁ ‹ ▁e ▁ › ▁and ▁/ e ː /, ▁ ‹ ▁e e ▁ › . ▁The ▁nas al isation ▁of ▁a ▁v owel ▁is ▁written ▁followed ▁by ▁an ▁n ; ▁for ▁example , ▁/ <0xE1> <0xBA> <0xBD> / ▁is ▁written ▁ ‹ ▁en ▁ › . ▁ ▁The ▁seven ▁v owel ▁sounds ▁may ▁also ▁be ▁either ▁length ened ▁/ i ː ▁e ː ▁ ɛ ː ▁a ː ▁ ɔ ː ▁o ː ▁u ː / ▁or ▁nas al ized ▁/ ĩ ▁ <0xE1> <0xBA> <0xBD> ▁ ɛ ̃ ▁ ã ▁ ɔ ̃ ▁ õ ▁ ũ /. ▁ ▁The ▁notation ▁of ▁t ones ▁was ▁recommended ▁in ▁ 1 9 7 3 , ▁but ▁in ▁practice ▁they ▁are ▁not ▁written . ▁The ▁trans cription ▁guide ▁published ▁in ▁ 2 0 0 3 ▁does ▁not ▁re iter ate ▁this ▁recommendation . ▁T ones ▁are ▁noted ▁sole ly ▁in ▁lex ic ographical ▁works . ▁However , ▁to ▁avoid ▁ambigu ity , ▁tone ▁mark ing ▁is ▁oblig atory ▁in ▁certain ▁cases . ▁ ▁For ▁example : ▁▁▁ ‹ ▁a ▁ › ▁he / she ▁( third ▁person ▁singular ▁pron oun ) ▁▁ ‹ ▁á ▁ › ▁you ▁( second