Heat the oven to 225° F and position a rack in the center or two racks in the upper and lower thirds of the oven.
Line baking sheet(s) with parchment paper.
Combine the egg whites and cream of tartar or vinegar in a clean dry mixing bowl.
If the egg whites are cold, set the bowl in a pan of hot water, or swirl the bowl over a burner, until they are room temperature or slightly warm to the touch.
Beat at medium to medium-high speed until the mixture is white and creamy looking -- no longer yellowish and translucent -- and holds a soft shape when the beaters are lifted.
Increase the speed to high and add the sugar a heaping teaspoon, one after the other -- without delay but without rushing.
If you are working with ½ cup of egg whites and 1 cup of sugar, it should take 2 to 3 minutes to add the sugar.
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