Assume MtextAutostack is set to on and the enabled autostacking is set to horizontal, you can use one of three codes to stack numerical dimensions a certain way. To stack with the numbers one on top of the other, use the slash (/) character (1/2"). To stack with a diagonal, use the hashtag (#) character (1#2"). To stack one on top of the other without a line, use the carrot (^) character (+.5^-.08).
To stack non-numerical text you use the same code above (X^Y), but it won't be stacked automatically. Next highlight letters or words on both sides of the ^ including the ^, right-click and pick Stack. The words will be stacked without a line. Use the other codes if you want to stack with a line or a diagonal line as described earlier.
You can change the way things are stacked by highlighting the stacked dimensions or words. A lightning bolt appears below the text -- pick that and a dialog box appears. Different options will be presented, the first two are Diagonal and Horizontal. Next is Unstack, and last is Stack Properties.
Stack Properties gives you the option to change the text content in the upper and lower portion of the stacked text. Below that are dropdowns for Style, Position, and Text size. Under Style there are four options -- Horizontal, Diagonal, Tolerance, and Decimal. The first two are obvious what they do, Tolerance and Decimal styles both stack horizontal without a line, with non-numerical text they both left-justify the text. With numbers, Tolerance will left-justify, and Decimal will justified on the decimal point (if there is a decimal point, otherwise it will be the same as Tolerance).
Position will adjust the stacked text vertically so that it is either top, center, or bottom justified (TStackAlign 0, 1, OR 2). Below that is Text size, which is a percentage of the mtext height (TStackHeight).
On the bottom rightof the Stack Properties dialog box are two more options, Defaults and AutoStack. With Defaults you can restore defaults or save your current changes. The AutoStack button is where you can enable autostacking and pick a horizontal or diagonal setting, and you can elect to have the leading blank removed after a fraction where 1 x/y becomes 1x/y as you type.
Notes from Cadalyst tip reviewer R.K. McSwain: Kent does a good job of explaining all of the details of stacking text in mtext.