The plug flow and mixed flow methods of modelling a screen are analysed in terms of screen length, internal fibre concentration and screen separation performance. Experimental data for consistency changes in screens of narrow length are incorporated into the analysis and from this data an equation for determining the real thickening in the screen and the fibre concentration profile along the screen is derived. Lastly, thicken equations are related to common equations used for modelling overall screen performance. The work directly links screen performance equations to passage ratio measurements from narrow length screen sections.