1) Chill the glass. You may want to fill it with ice, then empty it and refill, as some bartenders do with a martini glass.
2) Fill the glass with whole ice cubes. If you wish, take a wedge of lime and moisten the rim the glass with it.
3) Pour the gin over the ice, which should be cold enough that it crackles when the liquor hits it.
4) Fill glass almost to the top with tonic.
5) Squeeze one wedge of lime into the glass. Drop the squeezed lime into the drink as a garnish if you like; it’s not necessary, but can add a bit of extra flavor. (If you do, notes Dale DeGroff, make sure the peel has been washed.) Serve.