▁the ▁first ▁val ve ▁slide ▁in ▁a ▁position ▁where ▁it ▁may ▁be ▁manip ulated ▁by ▁the ▁music ian ▁with ▁the ▁left ▁hand ▁during ▁playing , ▁which ▁allows ▁tun ing ▁for ▁the ▁always - flat ▁ 5 th ▁partial ▁harm onic ▁at ▁the ▁C ▁below ▁middle ▁C . ▁ ▁Another ▁solution ▁for ▁eu phon ium ▁or ▁t uba ▁is ▁called ▁a ▁compens ating ▁system , ▁and ▁these ▁have ▁been ▁made ▁in ▁both ▁three - ▁and ▁four - val ve ▁versions . ▁ ▁In ▁a ▁compens ating ▁system , ▁the ▁tub ing ▁from ▁the ▁final ▁val ve ▁is ▁rout ed ▁back ▁to ▁the ▁first ▁val ve , ▁and ▁each ▁of ▁the ▁val ves ▁before ▁the ▁final ▁one ▁is ▁fitted ▁with ▁a ▁short ▁val ve ▁slide ▁in ▁addition ▁to ▁the ▁normal ▁ones . ▁ ▁When ▁the ▁third ▁or ▁fourth ▁val ve , ▁as ▁applicable , ▁is ▁de pressed ▁in ▁combination ▁with ▁any ▁of ▁the ▁other ▁val ves , ▁the ▁additional ▁tub ing ▁brings ▁the ▁pitch ▁into ▁t une . ▁ ▁This ▁has ▁the ▁primary ▁effect ▁of ▁making ▁the ▁lowest ▁oct ave ▁play able ▁including ▁B ▁natural ▁( un play able ▁on ▁non - comp ens ating ▁horn s ), ▁and ▁a ▁secondary ▁effect ▁of ▁putting ▁low ▁B ▁natural ▁and ▁C ▁natural ▁into ▁t une . ▁ ▁Comp ens ating ▁systems ▁add ▁weight ▁and ▁" st uff iness " ▁to ▁a ▁horn , ▁so ▁they ▁are ▁not ▁commonly ▁fitted ▁to ▁tub as . ▁They ▁find ▁principal ▁use ▁in ▁eu phon iums , ▁in ▁which ▁virt ually ▁all