Have you ever tried to make a straight line in your word document, or maybe even in Paint, and you discovered that your lines are wiggly, not straight? that happens sometimes. You can sometimes get it to straighten out by double-clicking on the beginning or end point of the line. But that doesn't always work depending on what platform you're in.
One thing you could do though that always work is, if you're using Microsoft word, or the Paint application's 'Shape Gallery' (shown above), right after you click on the 'Line Shape', before you start drawing, hold down your Shift Key, then start drawing. Do not release your Shift-Key until you are done drawing your line. This is guaranteed to give you those perfect line each time.
To make multiples of the same line, press the Ctrl + D Keys simultaneously while one of the original line is selected. (This only work in Microsoft Word, Excel & PowerPoint to duplicate shapes).