A break down of the image into a screen of dots of variable dimensions and frequency that makes it possible to simulate variations in grey and colour shades with a limited number of inks. Screen ruling, or frequency (measured in lines per centimetre or lines per inch) varies in accordance with the on-press performance of the paper: for example, newspaper printing uses a screen with a low frequency, of no more than 35 lines per centimetre or 85 lines per inch.