A double decrease is a decrease that decreases from three to one stitch and include sl1, k2tog, psso/sk2po (slipe one stitch, knit two stitches together, pass slipped stitch over), sl2, k1, psso/s2kpo (slip two stitches, knit one stitch, pass slipped stitches over), k3tog (knit three stitches together), sssk (slip three stitches knitwise (one at a time), insert left needle into the front of all three stitches and knit together). There are more but these are the most commonly used double decreases.
In the video below I use two charts - Chart A and B. Below are the two charts and chart key. Don't worry too much if you don't like charts, you don't really need to understand them to understand this tutorial.
You can also view the video here.
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Previous Monday Mini Tip posts and other tutorials can be found on my Tutorials page.