ZIP and RAR are two most popular archive formats on the Windows side of the planet. They are used to compress data like word documents and excel tables to take much less space.

This, in turn, is useful for generating efficient backups and sending one-year Microsoft financial reports by email.

People who are not very familiar with computers can understand the idea behind data compression rather easily. What I found almost impossible to explain to such people is why a file compressed by one archiver can be further compressed by another archiver.

Here is an idea how to illustrate the process of double compression.

Imagine that a file we are trying to compress is a sheet of paper. Let’s see how RAR archiver works. The goal here is to make the sheet of paper take less space.

Let’s say RAR compression is similar to folding a paper sheet in strips.

Our RAR-compressed file (folded piece of paper) takes much less space than the original and looks like this:

ZIP compression works a bit different from RAR but gives similar results. Let’s take an exact copy of the paper sheet we used for RAR compression.

One could say that this type of archiving is similar to rolling a sheet of paper into a tube.

The result again takes less space and looks like this:

Is there any room for improvement? Can the paper sheet be compressed further?

Of course. RAR archiver applied to a file “folds it” into strips. Who said that this file can not be another archive?

Our original file in ZIP archive is a sheet of paper folded into a tube. Here is how RAR compresses it further: folding into strips!

This is what a sheet of paper that was compressed twice, both by RAR and ZIP, looks like:

Archiving an original file, compressed by a RAR archiver, for the second time by ZIP will give exactly the same result:

In my experience, this explanation of the double compression by RAR and ZIP to people with limited computer experience works wonders.