### 4.3.3 Length and thickness of objects

Distances and lengths in LilyPond are generally measured in staff-spaces, the distance between adjacent lines in the staff, (or occasionally half staff spaces) while most `thickness` properties are measured in units of an internal property called `line-thickness.` For example, by default, the lines of hairpins are given a thickness of 1 unit of `line-thickness`, while the `thickness` of a note stem is 1.3. Note, though, that some thickness properties are different; for example, the thickness of beams is controlled by the value of the `beam-thickness` property, which is measured in staff-spaces.

So how are lengths to be scaled in proportion to the font size? This can be done with the help of a special function called `magstep` provided for exactly this purpose. It takes one argument, the change in font size (#-2 in the example above) and returns a scaling factor suitable for reducing other objects in proportion. It is used like this:

>     \new Staff ="main" {
>       \relative g' {
>         r4 g8 g c4 c8 d |
>         e4 r8
>         <<
>           { f8 c c }
>           \new Staff \with {
>             alignAboveContext = #"main"
>             \omit Clef
>             \omit TimeSignature
>             fontSize = #-2
>             % Reduce stem length and line spacing to match
>             \override StaffSymbol.staff-space = #(magstep -2)
>           }
>           { f8 f c }
>         >>
>         r4 |
>       }
>     }

Since the length of stems and many other length-related properties are always calculated relative to the value of the `staff-space` property these are automatically scaled down in length too. Note that this affects only the vertical scale of the ossia – the horizontal scale is determined by the layout of the main music in order to remain synchronized with it, so it is not affected by any of these changes in size. Of course, if the scale of all the main music were changed in this way then the horizontal spacing would be affected. This is discussed later in the layout section.

This, then, completes the creation of an ossia. The sizes and lengths of all other objects may be modified in analogous ways.

For small changes in scale, as in the example above, the thickness of the various drawn lines such as bar lines, beams, hairpins, slurs, etc does not usually require global adjustment. If the thickness of any particular layout object needs to be adjusted this can be best achieved by overriding its `thickness` property. An example of changing the thickness of slurs was shown above in Properties of layout objects. The thickness of all drawn objects (i.e., those not produced from a font) may be changed in the same way.

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#### LilyPond — Learning Manual v2.18.2 (稳定分支).