CRTs have two specifications for screen size: the CRT size (the actual size of the picture tube) and the viewable screen size (the usable screen area). Because the CRT picture tube is enclosed in the plastic casing, the viewable screen size is smaller than the overall CRT size. Though CRTs are commonly referred to by the picture tube size, it is the viewable area that is important in comparing a CRT to an LCD. Unlike CRTs, the viewing area of an LCD is the only valid measurement of its size. This is why many LCD specifications list a CRT equivalent size to identify the picture tube size that is required to achieve the same viewable size display.