What is the alto oboe?
From before the Baroque era through the classical period, the style of the oboe family of bass instruments was not established, and those instruments took a variety of forms and had a variety of names. At that time, an instrument equivalent to today's cor anglais (an F woodwind) was called the alto oboe. In older scores, the sounding pitch of the alto oboe is sometimes noted using an alto clef (as with a viola).
The English horn is another oboe relative in the alto range. The angle at which the pipe tapers is slightly different than that of the oboe, and the English horn is never referred to as an alto oboe. Comparing tones at the same pitch, the English horn produces notes that are more nasal and softer. Oboists also play the English horn, but it can be considered a separate instrument with a different feeling.