To me an odd ratio. Alternates below.
Option 1 – a one to one ratio of dry vermouth to either gin or vodka. For example 1.5 oz of each.
Option 2 – 2 to 1 ratio; as 2 oz. dry vermouth to 1 oz gin or vodka.
With either – add an olive OR lemon slice and perhaps a barspoon of lemon juice.
Both are stirred with ice in a mixing glass and and poured into a martini glass.
Never serve a martini on the rocks.