Combine egg whites and cream of tartar in mixing bowl. Beat until foamy, about 30 seconds. Continue beating on medium-high speed until egg whites form soft peaks, about 30 seconds. Gradually beat in sugar, 1 tablespoon at a time. Continue beating until stiff peaks form and meringue has a glossy look and sugar is dissolved, 3 to 4 minutes.
This recipe yields meringue for 1 (9-inch) pie.