When is it OK for composers to borrow from the works of other composers?

[aintgotnorhythm]

When is it OK for composers to borrow from the works of other composers? e.g. Stravinsky's Circus Polka contains a warped version of Schubert's Marche Militaire, Shostakovich's 15th symphony contains the tune from Rossini's William Tell Overture. I'm sure there are very many examples of this sort of thing, these were just a couple that came to mind. There are probably even more examples of composers incorporating the melodies from traditional folk songs. To what extent does a new composition have to be original?

[retrouvailles]

It's music. You can do whatever the hell you want.

[djealnla]

https://en.wikipedia.org/wiki/Musical_plagiarism

Also, check this out: 

[stevebob]

It's music. You can do whatever the hell you want.

Are you being facetious again?

[quantum]

It's music. You can do whatever the hell you want.

Agreed.

Also, check this out: 

Besides the obvious, I detect a bit of this:

[retrouvailles]

Are you being facetious again?

Again? How often am I facetious? I was being completely serious.

[pianist1976]

It's music. You can do whatever the hell you want.

I think that in general you are right.  

To what extent does a new composition have to be original?

At least in my country, there's a minimum number of bars to be identical to another music work in order to be considered plagiarism. I don't know exactly how many they are (and I also don't care a damn). In this cases I think that common sense must be the rule to be followed. Common sense is what will tell us if our work is a veiled homage, an explicit and conscious reference or just plain a sample of lack of creativeness. We cannot delude ourselves.

[ch101]

anyone remembered that ruberstein accused tchaikovsky of borrowing other composer's works for his piano concerto no1? and that was one of tchaikovsky's most famous works to this day.

[pianist1976]

anyone remembered that ruberstein accused tchaikovsky of borrowing other composer's works for his piano concerto no1? and that was one of tchaikovsky's most famous works to this day.

Who or what the hell is "ruberstein"? (you named him in at least two different threads)