This post describes how to work a yarn over. A yarn over (yo) is a single increase, which forms a hole in the knitted fabric. It is the main increase used in lace knitting.
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Ravelry: Barley worsted pattern by tincanknits
Regular and Reverse Yarn Over - The Easiest Way to Increase
Hat Knitting Patterns Using Sock Yarn – Knitting
Common Knitting Mistakes and How to Fix Them
front playing a hole knit
Aqua Love Note — Tanis Fiber Arts
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Ravelry: Flax worsted pattern by tincanknits
Lesson 10: 4 Easy Increases for Beginner Knitters - GoodKnit Kisses
Beginners Tip - How to Adapt Pattern Sizes from an Off-Gauge - YAK