Since the heyday of American West saloons in the late 1800s, scotch drinkers have been ordering their beverages by the finger. You might think this couldn't possibly be literal, given that most peoples' fingers differ in size; however; it is literal indeed. Be careful when ordering this way, as the finger used for measuring is that of the bartender's, so you'll want to scope out his hands first.
Lately, some enthusiasts have rallied to make 3/4 inch the official measure of a finger. This amounts to 1 ounce per finger, making for a fair pour no matter whose hands are in charge.
Measuring by the finger works best when using a classic tumbler -- also known as a rock glass.
Tips
If you'll be drinking scotch on ice, add ice after pouring the scotch.
With the glass set upright on a bar or table, wrap your fingers around it with your middle finger at the base of the glass above the solid bottom, where the liquid begins.
Pour the scotch up to the top of your middle finger for 1 finger, or keep going until it reaches the top of your index finger for a 2-finger measure.
Add ice if desired and enjoy.