"Trebles all round"

I have read that London journalists used to walk into a pub and announce: “Trebles all round.”

I have googled it, but I am still unclear why journalists would say “Trebles all round.”

Thank you,

James

Journalists are regarded as being very heavy drinkers. When you buy a serving of whisky or other spirit, there is a specific measured amount poured into a glass. Twice that amount would be ‘a double’, and here, the journalist asks for a treble - three times the amount of a single.
He might have walked up to the bar where a few of his friends/fellow journalists were drinking, and ordered a drink for all of them and himself by saying to the barman, “Trebles all round.”

(The ‘all round’ has become an idiom, but probably came from when people were sitting at a table, so that it was ‘a drink for everyone here at/around the table’.)

Thank you for the excellent explanation, Bazza.