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38.15.2 Specified Spaces

To display a space of specified width and/or height, use a display specification of the form (space . props), where props is a property list (a list of alternating properties and values). You can put this property on one or more consecutive characters; a space of the specified height and width is displayed in place of all of those characters. These are the properties you can use in props to specify the weight of the space:

:width width
If width is an integer or floating point number, it specifies that the space width should be width times the normal character width. width can also be a pixel width specification (see Pixel Specification).
:relative-width factor
Specifies that the width of the stretch should be computed from the first character in the group of consecutive characters that have the same display property. The space width is the width of that character, multiplied by factor.
:align-to hpos
Specifies that the space should be wide enough to reach hpos. If hpos is a number, it is measured in units of the normal character width. hpos can also be a pixel width specification (see Pixel Specification).

You should use one and only one of the above properties. You can also specify the height of the space, with these properties:

:height height
Specifies the height of the space. If height is an integer or floating point number, it specifies that the space height should be height times the normal character height. The height may also be a pixel height specification (see Pixel Specification).
:relative-height factor
Specifies the height of the space, multiplying the ordinary height of the text having this display specification by factor.
:ascent ascent
If the value of ascent is a non-negative number no greater than 100, it specifies that ascent percent of the height of the space should be considered as the ascent of the space—that is, the part above the baseline. The ascent may also be specified in pixel units with a pixel ascent specification (see Pixel Specification).

Don't use both :height and :relative-height together.

The :width and :align-to properties are supported on non-graphic terminals, but the other space properties in this section are not.

Note that space properties are treated as paragraph separators for the purposes of reordering bidirectional text for display. See Bidirectional Display, for the details.