What is alto clef?
I've never heard of it... and I think I should now somthing about it.
Can you guys describe what it is?

could you also tell me where it is used?

Alto clef is a clef not often used in music. It is shaped like a capital B and it is also known as a C clef. Middle C is found at the third line of the staff.

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Alto clef is currently used for the viola, the viola da gamba, and the alto trombone. Sometimes also used for the alto voicing and piano.

As stated earlier Alto clef is a C clef.
Just like Bass clef is an F clef.
With the F clef whatever line is between the two dots is F
With the C clef whatever line is between the "B" or the right angles is a C. Ask a tromboen player about tenor clef it's just one line higher.

French Horns also use the C clef.
A lot of pro-musicians will use Clefs to tranpose rather than trying to figure out each note. So IF you learn all your moveable clefs and practice them all it will be easier to transpose when you show up for a job and find out you have to cover someone elses part, or find out they wrote it wrong for your instrument.
John

i play trombone and i've used bass clef and tennor clef alot. i've never needed to learn alto clef.when you talk about alto clef you should also talk about tennor clef. they are very similar.

all tenor and alto clefs are are an easyer way to read and write music for intruments that play music way above the staf
like trombone, alto trombone, viola, exetra.

like john said it also makes if easy to transpose other istruments' parts to trombone. if i wanted to transpose a b flat instrument's mustic (trumpet, clarinet, sax...) i could pretend it's on tennor clef,then figure out what key itl'd be in, then i could play it.

my default picture is an example of a tennor clef