I've been getting this question a lot lately, so I thought a little explanation would be well suited to discuss on the ol' blog today.
There are several different ways that decrease sections in a knitting pattern are worded:
1. Repeat dec row every Xth row X more times. For example, "Dec Row (RS): K1, ssk, work to last 3 sts, k2tog, k1. Repeat Dec row every 10th row 2 more times."
2. Repeat dec row every X rows X more times. For example, "Dec Row (RS): K1, ssk, work to last 3 sts, k2tog, k1. Repeat Dec row every 10 rows 2 more times.
3. Dec every X rows (without indicating the original decrease row). Example, "Dec 1 st at each end of needle every 10 rows 3 times."
The trick to help you remember is that decreases are worked in SETS and the Dec Row + [The Number of Rows Indicated - 1] = a SET of decreases. So, continuing with our examples above, Dec Row + [10-1=9] = 10 rows total for a set.
In each of the examples above, 3 decrease rows are worked and there are 9 rows worked even between them. Here's how it looks all written out based on the examples above:
3 decreases have been worked over the correct number of rows (the number of rows is important for the length of the section based on row gauge). 10 rows for each set.
Now, here's the great part - because we know that the decreases are worked in sets (and we know how many rows are in each set), there aren't any hard and fast rules on WHERE in the set you have to place your decrease row. If you prefer it to be the last row of the set worked, then go for it. If you want it in the middle of the set, put it there. There aren't any detrimental effects to the taper or slope in this particular situation.
PS - This is the exact same way increase sets are worked, too.
