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#### 37.16.3 Pixel Specification for Spaces

The value of the `:width`, `:align-to`, `:height`, and `:ascent` properties can be a special kind of expression that is evaluated during redisplay. The result of the evaluation is used as an absolute number of pixels.

The following expressions are supported:

      expr ::= num | (num) | unit | elem | pos | image | form
      num  ::= integer | float | symbol
      unit ::= in | mm | cm | width | height

      elem ::= left-fringe | right-fringe | left-margin | right-margin
            |  scroll-bar | text
      pos  ::= left | center | right
      form ::= (num . expr) | (op expr ...)
      op   ::= + | -

The form num specifies a fraction of the default frame font height or width. The form `(num)` specifies an absolute number of pixels. If num is a symbol, symbol, its buffer-local variable binding is used.

The `in`, `mm`, and `cm` units specify the number of pixels per inch, millimeter, and centimeter, respectively. The `width` and `height` units correspond to the default width and height of the current face. An image specification `image` corresponds to the width or height of the image.

The elements `left-fringe`, `right-fringe`, `left-margin`, `right-margin`, `scroll-bar`, and `text` specify to the width of the corresponding area of the window.

The `left`, `center`, and `right` positions can be used with `:align-to` to specify a position relative to the left edge, center, or right edge of the text area.

Any of the above window elements (except `text`) can also be used with `:align-to` to specify that the position is relative to the left edge of the given area. Once the base offset for a relative position has been set (by the first occurrence of one of these symbols), further occurrences of these symbols are interpreted as the width of the specified area. For example, to align to the center of the left-margin, use

    :align-to (+ left-margin (0.5 . left-margin))

If no specific base offset is set for alignment, it is always relative to the left edge of the text area. For example, ‘`:align-to 0`’ in a header-line aligns with the first text column in the text area.

A value of the form `(num . expr)` stands for the product of the values of num and expr. For example, `(2 . in)` specifies a width of 2 inches, while `(0.5 . image)` specifies half the width (or height) of the specified image.

The form `(+ expr ...)` adds up the value of the expressions. The form `(- expr ...)` negates or subtracts the value of the expressions.

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