When you are deciding what type of fiber to use for your project, you will want to choose a fiber with the correct properties for your projects use.  If you are going to make a winter jacket, you will want a fiber that is warm and moves easily.  For a summer jacket, you will want a fiber that is cool.  Check out the table below for fiber properties.

You can also use this chart when deciding which fibers to blend.  You can blend fibers with different properties to create the result you are seeking.  A blend will use the advantages of one fiber to counteract the disadvantages of a different fiber.  You may want to blend some silk into your wool for the winter jacket which would utilize the warmth of wool plus the strength of silk for a warm and sturdy jacket.