In a large bowl, combine the flour, cream of tartar, baking soda and salt. Set aside.
In a large mixing bowl, beat 1½ cups of the sugar, butter, shortening and eggs on medium speed until well-blended.
Add the flour mixture to the mixing bowl, ½ cup at a a time, until the dough forms.
In a separate, small bowl, combine the remaining ¼ cup of sugar and the cinnamon. Stir until well-blended. Set aside.
Heat oven to 400 degrees.
Shape dough into 1¼-inch balls and roll in the sugar-cinnamon mixture, coating all sides of each ball.
Place the balls 2 inches apart on an ungreased cookie sheet.
Place cookie sheet on oven's middle rack and cook 8-10 minutes or until the cookies look set. (Note: For best results, use aluminum cookie sheets. If you are using a dark, nonstick pan, bake cookies at 375 degrees instead.)
Immediately move cookies from cookie sheet to cooling rack. The cookies will look a little 'raised' but will immediately deflate and firm up once cooled.
Recipe by Cooking is an act of love! at https://www.cookingisanactoflove.com/easy-snickerdoodles/