Technique
Fill the glass completely with hard ice, pour in the gin, and add well-chilled tonic down the side of the glass. One gentle stir is enough. More stirring releases carbonation and speeds melting without improving the mixture.
A gin and tonic is a highball cocktail made with gin and tonic water poured over a large amount of ice.

TonicDB editorial guide
A Gin and Tonic is simple enough that warm ingredients, weak ice, or flat tonic have nowhere to hide. The listed 1:2 ratio keeps the gin recognizable while giving the quinine bitterness and carbonation enough room to make a refreshing highball.
Fill the glass completely with hard ice, pour in the gin, and add well-chilled tonic down the side of the glass. One gentle stir is enough. More stirring releases carbonation and speeds melting without improving the mixture.
A highball is efficient and cold; a balloon glass gives botanicals more aromatic space. In either case, use more ice rather than less—an ice-packed glass stays colder and generally dilutes more slowly than a glass with a few floating cubes.
The most common mistake is opening a warm large bottle of tonic and using it across several drinks after it loses its fizz. Choose a freshly opened chilled mixer, and avoid piling on unrelated garnishes that obscure the gin rather than complement it.
Match one restrained garnish to the gin: lime for a bright classic profile, grapefruit peel for citrus-forward gin, or a rosemary sprig for a piney one. Change the garnish before changing the ratio so the base remains easy to evaluate.
These notes explain how to execute and adapt the recipe shown above. Ingredients and serving traditions vary, so taste and adjust thoughtfully.
Contributed by @alfg