There are many ways to note a decrease in a pattern. One way is to simply say to decrease over a certain number of stitches. For instance, “decrease 5 stitches over the next 50 stitches.” This means you need to decrease five times over a span of 50 stitches, but the pattern doesn’t tell you exactly where to put those five decreases--you have to do some math to figure out how to space them evenly. In this particular example, one decrease in every 10th stitch would work out perfectly.
Other ways you may see a decrease in a pattern include things like "decrease in every fifth stitch" or 4 sc, dec (4 single crochets, then one decrease), or to note the decreased stitch: 5 dc, 1 dc dec, 5 dc (five double crochets, then one double crochet decrease, and five more double crochets).
There has been a trend lately to write decreases as 2tog, which stands for “two together.” In other words sc2tog, “single crochet two together,” or dc2tog, “double crochet two together.” This is just a decrease over two stitches. You might even see sc3tog or dc3tog, which would be a decrease over 3 stitches. This has come up so often, I wanted to put it here so you would know it is just another way of saying the same thing.
Single and Double Crochet Decreases
Here's is a video demonstrating both the single crochet decrease and the double crochet decrease
To make a single crochet decrease:
Insert hk in st, yo, pull through (2 lps on hook)...
...insert hk in next st, yo, pull through (3 lps on hk)...
...yo, draw through all 3 loops.
You’ve just made one single crochet in two stitches.
To make a double crochet decrease:
Yarn over, insert hook into stitch indicated, yo, pull through stitch (3 lps on hk)...
...yo, pull through 2 lps, (2 loops on hk)...
...yo, insert hook into next stitch...
...yo, pull through stitch (4 lps on hk)...
...yo, pull through 2 lps (3 lps on hk)...
...yo, pull through all 3 lps.
You’ve just made one double crochet in two stitches.