My method for joining new yarn is super simple. You simply knit about 15 stitches with both strands held together. In other words, you hold the new and old yarn together in your hand and knit them together. 

On the next row, you can knit the double stitches together as one and cut off the ends.  

This method is super simple, quick, and nearly invisible. At the same time, the connection is completely secure and cannot come undone.

I'll explain exactly how to proceed, step by step. Here you'll learn a super-simple method that will stay with you forever—no more frustration with yarn joins!

The classic method for joining a new ball of yarn is usually a simple knot.

For these reasons, I don't think knotting the yarns together when changing yarn is a good idea:

• A knot may come undone, causing a hole
• Knots can be noticeable to the touch
• Knots can cause unsightly bulges
• Knots can shift to the right side of the knitted piece and become visible

For these reasons, I avoid knots. If I come across a production-related knot within a ball of yarn, I usually undo it and also use my double-strand method.

Step 1: 

Let the existing strand run out to about 30 cm. Make sure the end is not too short so that you can comfortably knit about 15 stitches with it.

Step 2: 

Add the new strand. Make sure the beginning of the strand overlaps securely. Place the new strand alongside the old one and pick up both together with your left hand.

Step 3: 

Knit about 15 stitches with both strands held together. To do this, knit with the new and old strands simultaneously as if they were a single strand—in other words, held double. 

Step 4: 

Knit the new stitches once on the next row or round. Each double stitch is knitted as one stitch.

Step 5: 

Now you can simply cut off the excess ends. Be careful not to cut too close; I usually leave about one centimetre to avoid accidentally damaging the existing stitches.

I use my double-knitting method with almost all yarns. It is particularly suitable for very thin and fine yarns, as the join is so smooth. 

I have also used the double-knitting method with very thick yarns; in this case, a slight thickening is visible in the double-knitted area, but it is barely noticeable.

For single-colour knitted pieces when changing balls of yarn, I always use the double-knitting method. 

The only exception for me is colourwork projects where the colour changes. Since the change has to happen at exactly the right point, tying a knot is unavoidable. It works well to make a slipknot around the main yarn. Since both yarns continue, this knot does not cause any problems with gaps.