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The total height of each display line consists of the height of the contents of the line, plus optional additional vertical line spacing above or below the display line.
The height of the line contents is the maximum height of any character or image on that display line, including the final newline if there is one. (A display line that is continued doesn't include a final newline.) That is the default line height, if you do nothing to specify a greater height. (In the most common case, this equals the height of the corresponding frame's default font, see Frame Font.)
There are several ways to explicitly specify a larger line height, either by specifying an absolute height for the display line, or by specifying vertical space. However, no matter what you specify, the actual line height can never be less than the default.
A newline can have a line-height text or overlay property
that controls the total height of the display line ending in that
newline.
If the property value is t, the newline character has no
effect on the displayed height of the line—the visible contents
alone determine the height. The line-spacing property,
described below, is also ignored in this case. This is useful for
tiling small images (or image slices) without adding blank areas
between the images.
If the property value is a list of the form (height
total), that adds extra space below the display line.
First Emacs uses height as a height spec to control extra space
above the line; then it adds enough space below the line
to bring the total line height up to total. In this case, any
value of line-spacing property for the newline is ignored.
Any other kind of property value is a height spec, which translates into a number—the specified line height. There are several ways to write a height spec; here's how each of them translates into a number:
(face . ratio)nil which means a ratio of 1.
If face is t, it refers to the current face.
(nil . ratio)Thus, any valid height spec determines the height in pixels, one way or another. If the line contents' height is less than that, Emacs adds extra vertical space above the line to achieve the specified total height.
If you don't specify the line-height property, the line's
height consists of the contents' height plus the line spacing.
There are several ways to specify the line spacing for different
parts of Emacs text.
On graphical terminals, you can specify the line spacing for all
lines in a frame, using the line-spacing frame parameter
(see Layout Parameters). However, if the default value of
line-spacing is non-nil, it overrides the
frame's line-spacing parameter. An integer specifies the
number of pixels put below lines. A floating-point number specifies
the spacing relative to the frame's default line height.
You can specify the line spacing for all lines in a buffer via the
buffer-local line-spacing variable. An integer specifies
the number of pixels put below lines. A floating-point number
specifies the spacing relative to the default frame line height. This
overrides line spacings specified for the frame.
Finally, a newline can have a line-spacing text or overlay
property that can enlarge the default frame line spacing and the
buffer local line-spacing variable: if its value is larger than
the buffer or frame defaults, that larger value is used instead, for
the display line ending in that newline.
One way or another, these mechanisms specify a Lisp value for the spacing of each line. The value is a height spec, and it translates into a Lisp value as described above. However, in this case the numeric height value specifies the line spacing, rather than the line height.
On text terminals, the line spacing cannot be altered.