Runways are normally designated by rounding the magnetic alignment to the nearest 10 degrees (i.e. a direction of 238, 241 or 243 will become runway 24). Magenetic variation is not constant but varies, sometimes over quite short distances (although not in the UK). The combination of true direction of the centreline and the local magnetic variation and the rounding of designators means that designator changes appear to be almost random. In reality, because the designator is only an approximation of the direction of the runway, changes are sometimes deferred for operational or administrative reasons, until a planned repainting of the markings is carried out or some new flight procedures are published, for example.