sl1, k2tog, psso (or SK2P) – left-slanting double decrease
sl1, k2tog, psso is an abbreviation for slip one, knit two together, pass one over. Slip one stitch as if to knit. Insert right needle through 2 stitches at once from left to right and complete knit stitch as usual. Pass your slipped stitch over the single stitch left after knitting the two stitches together. This creates a left-slanting double decrease, decreasing from three stitches to one stitch.
sl2, k1, p2sso (or S2KP2) – centered double decrease
sl2, k1, p2sso is an abbreviation for slip two, knit one, pass two over. Slip two stitches together as if to knit, knit one stitch, then pass the two slipped stitches over the stitch you just knit. This results in a centered double decrease.
K3tog –right-slanting double decrease
K3tog is an abbreviation for knit three together. It involves knitting three stitches together as though they were one stitch. Completing a k3tog creates a double decrease in your knitting because where once there were three stitches; there is now only one stitch. This decrease slants to the right. To work a k3tog, insert your right needle through 3 stitches at once from left to right and complete the knit stitch as usual.