or izes ▁a ▁shift ▁of ▁* u ▁to ▁ ▁med ial ly ▁in ▁W ich ita , ▁but ▁does ▁not ▁have ▁enough ▁examples ▁to ▁fully ▁analyze ▁all ▁the ▁possible ▁environments . ▁He ▁also ▁discuss es ▁a ▁potential ▁shift ▁from ▁* a ▁to ▁, ▁but ▁again , ▁does ▁not ▁have ▁enough ▁examples ▁to ▁develop ▁a ▁definit ive ▁hypothesis . ▁Taylor ▁finds ▁ ▁only ▁occurs ▁with ▁inter v ocal ic ▁gl ott al ▁stops . ▁ ▁Ro od ▁arg ues ▁that ▁ ▁is ▁not ▁ph on em ic , ▁as ▁it ▁is ▁often ▁equivalent ▁to ▁any ▁v owel ▁+ ▁ ▁+ ▁any ▁v owel . ▁For ▁example , ▁ ▁is ▁frequently ▁contract ed ▁to ▁ ▁( the ▁high ▁tone ▁is ▁an ▁effect ▁of ▁the ▁el ided ▁con son ant ). ▁There ▁are ▁relatively ▁few ▁cases ▁where ▁speak ers ▁will ▁not ▁accept ▁a ▁substitution ▁of ▁v owel ▁+ ▁ ▁+ ▁v owel ▁for ▁; ▁one ▁of ▁them ▁is ▁ ▁' e agle '. ▁ ▁Ro od ▁also ▁propos es ▁that , ▁with ▁three ▁v ow els ▁that ▁are ▁arg u ably ▁high , ▁mid , ▁and ▁low , ▁the ▁front - back ▁distinction ▁is ▁not ▁ph on em ic , ▁and ▁that ▁one ▁may ▁therefore ▁speak ▁of ▁a ▁' vertical ' ▁v owel ▁invent ory ▁( see ▁below ). ▁This ▁also ▁has ▁been ▁claimed ▁for ▁relatively ▁few ▁languages , ▁such ▁as ▁the ▁North west ▁C au cas ian ▁languages ▁and ▁the ▁N du ▁languages ▁of ▁Pap ua ▁New ▁Guinea . ▁ ▁There ▁is ▁clearly ▁at ▁least