In our family, gin and tonic can only be drunk when you have a bath, so we call it "Bath water". We have even procured our own bathing water glasses, for the purpose.
If you want ice in the drink, fill the glass 3/4 full with it.
Hell i 4 cl gin
Fill the rest of the glass with tonic (approx. 1.5 dl) - it must be a tonic of good quality.
Cut a boat of lime, rub it around the edge of the glass, and place it in the glass.
Stir lightly with a drinking stick and serve.
I have taken some information from Wikipedia about the origins of GT:
GT is said to be British officials in India's need to ingest the quinine-containing tonic, because quinine is a cure for malaria. Since early versions of tonic did not taste particularly good, they looked for something that made it easier to absorb, and found that by mixing it with gin, the unpleasant taste was neutralized. Gin and tonic is one of the few drinks where the liquor originally served as a mixing water.