If you don't feel like reading all of the following stuff, just skip to the last paragraph.
First of all, it makes the paper piecing process much easier and the pattern preparation much faster.  When I first developed my method of paper piecing I used regular copy paper because I wanted to eliminate the need to purchase special papers.  However, in the early years I experimented with all different kinds of paper and discovered 17lb vellum.  It was perfect for the process.  And when I taught live classes I supplied the vellum patterns as part of the class materials.

Then came virtual classes and you all needed to print your own patterns.  Copy paper is easily available and almost everyone has copy paper.  It works, but there's more pattern prep to do because copy paper isn't smooth and isn't translucent.  For most of my patterns, copy paper is just fine because the patterns don't have lots of segments.  For instance, "Not Your Daddy's Log Cabin" blocks have twenty-two segments and regular copy paper doesn't quite cut it.  It's usable, but not ideal.  Thus, the need for vellum.

You can look for your own 17# vellum, but it's not that easy to find.  The links I've supplied are brands that I've used before and I can definitely say that they are my favorites.  They are also delivered within a couple of days, depending upon where you live.  If you choose to look for your own, you must make sure it's17# (pound), and measures 8-1/2" x 11" .  Also, if you've never taken one of my classes before, you'll just have to trust that whichever quantity you order, one sheet of paper will go a very long way.  
So the bottom line is this...get the vellum.  It goes through the printer the same as copy paper.  You'll find streamlines the paper-piecing process because it's smooth and translucent, thus eliminating time.  Set your printer for "Best" quality printing and give the ink a minute to dry before you start handling the patterns.   

Here are links  for 100 sheets, 50 sheets or 30 sheets.  The 100 sheet option is the most economical, and order early to make sure you have your vellum in time for class.

17lb Translucent Vellum 8 1/2" x 11"*

To order 100 sheets click 

To order 50 sheets click 

To order 30 sheets click

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*  Usually not available in stores and will need to be ordered online