The bass oboe or baritone oboe is a double reed instrument in the woodwind family. It is about twice the size of a regular (soprano) oboe and sounds an octave lower; it has a deep, full tone somewhat akin to that of its higher-pitched cousin, the English horn. The bass oboe is notated in the treble clef, sounding one octave lower than written. Its lowest note is B2 (in scientific pitch notation), one octave and a semitone below middle C, although an extension may be inserted between the lower joint and bell of the instrument in order to produce a low B♭2. The instrument's bocal or crook first curves away from and then toward the player (unlike the bocal/crook of the English horn and oboe d'amore), looking rather like a flattened metal question mark; another crook design resembles the shape of a bass clarinet neckpiece. The bass oboe uses its own double reed, similar to but larger than that of the English horn.
Taken from https://en.wikipedia.org/wiki/Bass_oboe