What is "mean line"

Mean \Mean\, a. [OE. mene, OF. meiien, F. moyen, fr. L. medianus that is in the middle, fr. medius; akin to E. mid. See Mid.]

1. Occupying a middle position; middle; being about midway between extremes.

   Being of middle age and a mean stature.
   --Sir. P. Sidney.

2. Intermediate in excellence of any kind.

   According to the fittest style of lofty, mean, or lowly.
   --Milton.

3. (Math.) Average; having an intermediate value between two extremes, or between the several successive values of a variable quantity during one cycle of variation; as, mean distance; mean motion; mean solar day.

   Mean distance (of a planet from the sun) (Astron.), the average of the distances throughout one revolution of the planet, equivalent to the semi-major axis of the orbit.

   Mean error (Math. Phys.), the average error of a number of observations found by taking the mean value of the positive and negative errors without regard to sign.

   Mean-square error, or Error of the mean square (Math. Phys.), the error the square of which is the mean of the squares of all the errors; -- called also, mean square deviation, mean error.

   Mean line. (Crystallog.) Same as Bisectrix.

   Mean noon, noon as determined by mean time.

   Mean proportional (between two numbers) (Math.), the square root of their product.

   Mean sun, a fictitious sun supposed to move uniformly in the equator so as to be on the meridian each day at mean noon.

   Mean time, time as measured by an equable motion, as of a perfect clock, or as reckoned on the supposition that all the days of the year are of a mean or uniform length, in contradistinction from apparent time, or that actually indicated by the sun, and from sidereal time, or that measured by the stars.

In typography, the mean line, also called the midline, is half the distance from the baseline to the cap height. This may or may not be the x-height, depending on the design of the lower case letters. A very high or very low x-height may mean that the midline is above or below the x-height.

This confusion has been perpetuated by books, and websites, on typography which copied from books on type back to the original book or source on type that first failed to clarify this difference.

Round glyphs will break ( overshoot) the mean line slightly in many typefaces, since this is aesthetically more pleasing; a rounded shape will appear visually smaller than flat-topped (or bottomed) shapes of equal height, due to an optical illusion.