I posted to my Instagram a step by step on how to make a 4 bar envelope with a Paper Source envelope template...one of the steps was to use a Paper Tearing ruler to create a faux deckled edge at the pointed flap...I received so many comments, DMs, and emails regarding that ruler, I had no idea that it was not a well known and used tool. I will answer all the questions and give you the biggest tip in using a Paper Tearing Ruler here...
Where did you get it/What kind is that? I purchased mine quite some time ago, from amazon.com, the brand is Plaid, and unfortunately it is no longer available, but a quick search results in several other brands. I suggest getting one with smaller "teeth" as it will result in a more "natural" edge. Google "paper tearing ruler" or "deckled edge tool" as well. They are out there!
Is it sharp? Will you cut yourself?: The teeth are sharp enough to tear paper but not sharp enough to cut your skin, I have never injured myself using it, but please take caution as you would while using any tool.
TIP: to get the most "natural" looking tear, place the ruler on top of the side you want to tear off, and tear against the ruler. There will be two edges, one will have a slightly raised ridge that is lifted because of the ruler. You don't want that ridge on your finished paper side. Hopefully the images below help communicate this. Click to enlarge and read the captions.
I have used this paper tearing ruler on handmade paper, watercolor paper, text weight paper, and cover weight paper, all creating a faux deckled edge at the size of paper that I need. Nothing beats a natural deckled edge on handmade paper, but sometimes you just need something at a certain size, and this ruler is a tool that I always reach for.
Hope this helps you in your future paper tearing adventures!