Have egg whites at room temperature to obtain greatest volume. Place them in a medium bowl with cream of tartar, salt and vanilla.
Beat with electric or hand beater, at medium speed, until entire mixture is frothy. Do not beat until eggs stiffen.
Add sugar, a little at a time, beating well after each addition. Do not underbeat. Beat until sugar dissolves to help prevent beading (those brown syrup drops on top). To test, rub some of the meringue between your fingers to see if it’s still grainy. (The grains are undissolved sugar.) Continue to beat until stiff, pointed peaks form when you lift beater slowly.
Place spoonfuls of meringue around edge of pie filling, spreading it so it touches inner edge of crust to seal all around. This prevents shrinkage. Pile remainder of meringue in center of pie and spread to meet meringue around edge. If the filling is not covered completely, the oven heat may cause it to weep. (Stirring the cooked filling may cause it to weep; water will collect under the meringue.) Lift up meringue over pie in points with back of teaspoon.
Bake in moderate oven (350°) 12 to 15 minutes, or until meringue peaks are golden brown. Too long baking may cause weeping. Cool gradually away from drafts.
With egg whites plentiful in farm kitchens, many country pie bakers like to use 5 egg whites and 10 tblsp. sugar to make a high cover on their 9 and 10" pies.