The blue lines at the top and to the left of the grid are guidelines. There are six horizontal and six vertical guidelines. They provide additional visual reference points.
Use Move Guidelines 
to move the guidelines and baseline. Click Move Guidelines and the pointer changes to the double-arrow pointer 
. Click and drag the guideline to move it to the desired position. The guidelines always snap to the dots of the Font Grid.
Shortcut
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Font Page:
The guidelines are initially docked at the top and to the left of the grid. Place the horizontal guidelines to give positions for the cap height, x height and descender depth. You may also wish to use a horizontal guideline to show the height of your start and end stitches, which is particularly useful for continuous fonts. Use the vertical guidelines to mark the right edge of characters that are the same width (for instance, 'b', 'd', 'p').
The same guideline positions are used for the whole font or SuperDesign set. The same height and width can therefore be used for each similar character. For instance, 'a' and 'u' would normally be the same height and width, while 'm' and 'w' would be wider.