Shops and other businesses

Shops and other businesses that are regular features in towns or cities can be used with the definite article when you do not want to pick out a particular one.

He might have been to the barber's to please his mother.

He's at the dentist's.

It enables you to put money into the bank and withdraw it.

Mother sent me to the butcher’s to get a nice joint of beef.

Some words like this are: baker's, bank, barber's, dentist's, doctor's, greengrocer's, hairdresser's, post office, pub.

In these cases the activity is as important as the place. You go 'to the post office' to get some stamps, 'to the bank' to get some money, 'to the barber's/hairdresser's' to have your hair cut, 'to the dentist's' to have your teeth filled, and so on.

If you want to have a drink you can say 'Let's go to the pub' without having a particular one in mind. But of course the difference is not always clear, or important.

When he got tired of painting he went to the pub.

This could mean one particular pub that we know about, or any pub; the important thing is that he needed a drink or some company.

Musical instruments

When you are talking about someone's ability to play a musical instrument you use the definite article.

She plays the guitar. I became interested in the piano again.

This does not mean one particular guitar or piano, as an object; we are talking about the ability. So the second example above means '...interested in playing the piano ...'. If you want to talk about roles in a musical group or in a piece of music you use no article.

...making up an unusual trio of trumpet, guitar, and drums.

I don't think I know of any duets for piano and trumpet.

There is also no article after 'on', meaning 'playing'.

...a recording featuring Harris's old pal Ray Brown on bass.