I'm changing this to apply only to musical theatre - opera works should follow the classical guidelines when/if those are created, while theatre is its own beast currently.

Agreed. It also doesn't cover musical theatre.

It didn't, really - those guidelines are just for tracks. I mean, I would personally use a reasonably similar style for works too, at least for opera, but

It seems like this may have been resolved by the new Classical guidelines (see STYLE-344). If not, please clarify...

Ah, I didn't realize Opera was already covered by Classical. I was reading https://wiki.musicbrainz.org/Style/Specific_types_of_releases/Opera which makes no mention of works. At the very least that should be fixed to refer to https://wiki.musicbrainz.org/Style/Classical/Works#Opera for works.

The current guideline says to have Opera, Act and Number works, and that extracts for arias performed often can be added when there are no numbers. That doesn't sound that bad to me, but I'm not an opera expert

Currently we get common splits while also getting many unwanted uncommon splits. Many of these works start middle of another and end middle of another so there's no way to link them as part of anything else but the main work. I stopped adding opera parts when I understood that I would be adding hundreds of works which would have only one recording linked with it. Maybe guideline could limit adding works like this.

It's frustrating when titles don't match but to me it's even more frustrating seeing many similar work titles and having no idea which one to select. For some reason it's important for opera to have matching work for every recording. If I would split one movement symphony to 20 parts because of the same reason editors wouldn't agree with me. We commonly use partial attribute with main work when there isn't matching works or good arguments for creating new works.

It's common that operas have different versions and sometimes director makes smaller changes to libretto. Because of these variations we have some works which can't be linked with others. If director once changed some lyrics it shouldn't mean that it's now considered being a new work.

We guide people not adding new work for work arrangement when there isn't enough recordings of it. We could easily have similar type of rule with opera recordings and part works. For example we could guide for not adding part work if it hasn't been recorded at least twice. In cases of not finding suitable work recording could be linked with act, scene or main work.

Another point: Apparently the libretti are not always consistent with each other, resulting in people entering bogus tracklistings such as http://beta.musicbrainz.org/release/e15a3c82-11e6-44d0-996c-47ca1f460c65 (see annotation for the explanation.)

The "take a random part and use some first words" is because of the opera guideline "The name of the song is usually the first words of the part; enclosed in double quotes. The exact words used can differ from one release to another. Follow what the printed release says."

This does have the advantage that we can eventually get down to a set of common splits that are used on many different releases, and then we can add aliases and knowledgeable editors can merge the part-works where appropriate.

I don't think there is any good choice but to follow the releases' tracklistings. There are several reasons for this, but among them: Some operas do not have good online sources available; they're often in a language not spoken by the editor, and translations don't help either; the libretti don't always have clear breaks into parts either; trying to match up a tracklisting where the work titles don't match is just an exercise in frustration.

There must be hundreds of ways to split Don Giovanni. I don't know which one would be the most suitable for MusicBrainz. Currently it seems to be fine to just take random part and use some first words of it as title. This makes no sense.

Let's use libretto text "this is example libretto" as an example. If I have full recording of this text titled "this is" I can't really assume that it's enough to link it with a work with identical name. If someone earlier created works "this is" and "example libretto" I should know to link both of them to my recording (or maybe create parent work). But how could I know that I should link also "example libretto" when it's not mentioned on release track list? Most of the editors don't care to check the libretto text for something like this. Same work might now have recordings with durations from 1 minute to one hour. Currently same work could mean different things.

Instead of using work titles for storing character names we could use disambiguations or work attributes. Maybe character names could be included on tracklist (like on release) but not with works titles.