Mac OS X Lion: The Missing Manual

How Documents Know Their Parents

Every operating system needs a mechanism to associate documents with the applications that created them. When you double-click a Microsoft Word document icon, for example, it’s clear that you want Microsoft Word to launch and open the document.

So how does Mac OS X know how to find a document’s mommy?

It actually has two different mechanisms.

• File name extensions. A file name extension is a suffix following a period in the file’s name, as in Letter to Mom.doc. (It’s usually three letters long, but it doesn’t have to be.) These play a role in determining which documents open into which programs.

  GEM IN THE ROUGH: Using the Dock or Sidebar for Drag-and-Drop

  The Mac is smart about the relationship between documents and applications. If you double-click a TextEdit document icon, for example, TextEdit opens automatically and shows you the document.

  

  But it’s occasionally useful to open a document using a program other than the one that created it. Perhaps, as is often the case with downloaded Internet graphics, you don’t have the program that created it, or you don’t know which one was used. This technique is also useful if you want to open a Read Me file into your word processor, such as Word, instead of the usual TextEdit program.

  In such cases, the Dock is handy: Just drag the mystery document onto one of the Dock’s tiles, as shown here. Doing so forces the ...