▁a ▁reference ▁for ▁fourth ▁position , ▁but ▁this ▁is ▁not ▁accessible ▁during ▁fast ▁pass ages ▁and ▁is ▁mass ively ▁un re li able ▁as ▁in ton ation ▁var ies ▁by ▁instrument ▁make ▁and ▁model . ▁Art ic ulation ▁is ▁also ▁an ▁important ▁consideration ▁when ▁playing ▁trom bone : ▁im prec ise ▁art ic ulation ▁may ▁add ▁unw anted ▁gl iss and i ▁to ▁a ▁note ▁not ▁requiring ▁it . ▁Im prec ise ▁art ic ulation ▁often ▁ends ▁up ▁as ▁poor ▁in ton ation ▁and ▁tone . ▁ ▁Wood w inds ▁are ▁manufact ured ▁with ▁holes ▁that ▁must ▁be ▁covered ▁or ▁un cover ed ▁to ▁short en ▁or ▁length en ▁the ▁bore ▁to ▁change ▁pitch , ▁and ▁( except ▁for ▁fl ute ) ▁require ▁register ▁keys ▁to ▁change ▁oct aves . ▁ ▁Most ▁wood wind ▁instruments ▁comp ly ▁to ▁a ▁regular ▁harm onic ▁series ▁of ▁pitch es ▁and ▁require ▁one ▁register ▁key ▁to ▁change ▁oct aves . ▁The ▁exception ▁is ▁the ▁clar inet , ▁which ▁sounds ▁only ▁every ▁other ▁pitch ▁of ▁the ▁harm onic ▁series . ▁This ▁means ▁that ▁the ▁clar inet , ▁rather ▁than ▁having ▁an ▁oct ave ▁key , ▁has ▁a ▁" register ▁key " ▁that ▁ra ises ▁the ▁pitch ▁by ▁a ▁ 1 2 th . ▁ ▁Br ass ▁instruments ▁with ▁val ves ▁have ▁an ▁inher ent ▁in ton ation ▁defect ▁in ▁that ▁val ve ▁combinations ▁tend ▁to ▁be ▁sharp . ▁This ▁is ▁because ▁the ▁open ▁horn ▁is ▁in ▁a ▁pitch ▁of ▁( for ▁example ) ▁B , ▁so ▁de press