They do! Once you play something enough you can basically do it by memory, but the pieces have so many moving parts that it's important to keep your place with everyone. Plus pieces can be LONG and there is a huge chance of stumbling and getting off if you don't have something to keep you on time

Yes they do. Primary violinists will almost never have sheet music in front of them - then you have Pianists that will play an entire repertoire without any sheet music either.

I’ll dig it up but there’s this an amazing pianist that can play a whole hour of Liszt using only memory.

There are a billion notes and no room for "rock and roll" sloppiness

Soloists usually do, and often the rest of the orchestra also has, but has the music there anyway.

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And I've seen more than a few performances where the orchestra has played the whole thing together maybe twice prior to the performance you're watching.

Because it’s very complex music and the pieces are very long. Little bit different from a pop song with four chords that repeat over and over.

Unlike a modern band, an orchestra or classical ensemble usually doesn’t have a particular set of songs that it knows. It is expected to perform a variety of pieces throughout the entire spectrum of world music.

If, for example, you had an orchestra memorize every symphony it played you’d have a massive decrease in the number of pieces it can perform each year in exchange for the unclear benefits of memorization.

Have you tried memorizing 1-1/2-2 hours of extremely complex music? They would have to do this every week during the performance season. They also practice several hours a day as well as teach at nearby universities. It's a full schedule and memorizing really doesn't add anything to the performance.

While some orchestras and ensembles I play in rehearse once a week, usually when I get hired to play for an orchestra, it's two rehearsals and a concert. Add that to the other groups I play in, and it's just not worth it to memorize music. If ai did show up to an orchestra concert with my parts memorized, I think the only thing I'd do is piss of the other musicians around me. And my intonation usually takes care of that.

But sure, try memorizing the first bassoon part to the Verdi Requiem.

Players in jazz swing and big bands often use sheet music as well. The more complex the music and the more people playing simultaneously tend to make having sheet music more reliable than memory. Most studio work is off something in writing, whether its lead sheets or full transcriptions. Saves rehearsal time and wasted energy to have written music.

I played classic guitar when I was a teen. I did memorise the music, but you’re not allowed to make mistakes when performing, unlike say rock music. Which is why I kept track of what I was playing during the performance. It also helps to distract your attention from the audience.

Because that would be absurd. If they do happen to memorize it, it's either because they've played it thousands of times or have a superior memory.

Depends on what the music is & how long they've had to practice it. If they're career musicians for hire, they could go thru hundreds of pieces of music for various different gigs and not have the time to memorize everything. But if it's a soloist and it's their own music, they likely will have memorized a piece that they wrote.

In my experience (French horn player here) you do sort of memorize the important bits but during rehearsal you make performance notes on the sheet music (with a pencil!) and use it as a guide during performance

You think they don't?

Why do bricklayers follow the architects plans? Why don’t they build houses by heart?

No need. It’s all on the page.