Depending on your needs, you may want fractions displayed as stacked horizontally or stacked on the diagonal. With AutoStack enabled, if you are entering fractions using the divide, or slash key “/”, then as soon as you complete the fraction and press the space bar, the numbers will change. At this moment you will see a little icon with a lightning bolt in it. Stop and click this icon for your choices. See below.
If you would like to disable the automatic stacking of fractions, see Disabling AutoStack.