A red-line drawing shows corrections or changes to a previous version. The term red line literally comes from the (typically) red pen used to make hand-written changes to a paper drawing. Today, architects and engineers use digital pens to make changes to digital drawings, but the color used is still usually red.

Red lines are essentially the layers of changes that a design goes through in order to reach its final form. Hence, going periodically through these red lines can help you keep track of exactly what changes have been made in the design since it was started.

"Redlining" is a form of editing in which red ink is used to indicate removal or addition of text in Microsoft Word. You can redline a Microsoft Word document by using Microsoft Word's built-in "Track Changes" feature, or you can manually redline the document with font color changes and strike-throughs.