The Oehler system is a system for clarinet keys developed by Oskar Oehler. Based on the Müller system clarinet, the system adds tone holes to correct intonation and acoustic deficiencies, notably of the alternately-fingered notes B♭ and F. The system has more keys than the Böhm system, up to 27 in the Voll-Oehler system. It also has a narrower bore and a longer, narrower mouthpiece leading to a slightly different sound. It is used mostly in Germany and Austria. Major developments include the patent C♯, low E-F correction, fork-F/B♭ correction and fork B♭ correction. Fingering charts can be found for example in this reference.