电视的画面清晰度是以水平清晰度作为单位，电视屏幕由电子枪射出的电子，经由磁场偏向后打在屏幕上而发光，因此每一个图框都由电子枪的扫描线画出来。一个图框的扫描线数是525条或625条，美国国家电视系统委员会（National Television System Committee，NTSC）制订的电视画面播映标准，明定每幅画面必需具备五百二十五条扫描线，及每秒钟必需播映三＋幅画面，而欧洲国家则有每个图框625条扫描线，且每秒25个图框的电视系统。通俗地说，我们可以把电视上的画面以水平方向分割成很多很多扫瞄线，分得越细，这些画面就越清楚，而水平线数的扫瞄线数量也就越多。清晰度的单位是「电视行（TVLine）」也称线。意思是从水平方向上看，相当于将每行扫瞄线竖立起来，然后乘上4：3或者16：9的宽高比，构成水平方向的总线数。

A scanline is one line, or row, in a raster scanning pattern, such as a line of video on a cathode ray tube (CRT) display of a television set or computer monitor.

On CRT screens the horizontal scan lines are visually discernible, even when viewed from a distance, as alternating colored lines and black lines, especially a progressive scan signal with below maximum vertical resolution is displayed. This is sometimes used today as a visual effect in computer graphics.