In some cases you are "required" to tip when you purchase some service or some thing.
Here are some of the cases.

Restaurant / Bar / Pubs

You're usually invited to leave a tip to the server when you dine. It usually amounts to the 15 % of the total. It can be higher, if you're particularly satisfied by the service, lower if you're not. If you're paying by cash, you just have to leave the amount you want (or you can pay, get your change, then leave the tip). If you're paying with the credit card, you will get your bill, then you'll give the waiter the credit card and he or she will come back with the credit card receipt. In the receipt there is a dedicated space to write your tip.

Sometimes the tip is already included, and this should be clearly indicated on th receipt (see the second example, below). Pay attention because sometimes it's only written in the menu ("service included"), or you have to check on the receipt if it's a "tip" or an "additional tip" or "additional gratuity".

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