A gin and tonic or, less frequently, gin tonic is a highball cocktail made
with gin and tonic water poured over a large amount of ice. The ratio of
gin to tonic varies according to taste, strength of the gin, other drink
mixers being added, etc., with most recipes calling for a ratio between
1:1 and 1:3. It is usually garnished with a slice or wedge of lime. To
preserve effervescence, the tonic can be poured down a bar spoon. The ice
cools the gin, dulling the effect of the alcohol in the mouth and making
the drink more pleasant to taste. (Wikipedia)
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My favorite is a simple Gin and Tonic with a splash of lemon and rosemary.