Mermaid blankets are clearly awesome. You could also call this a fishtail blanket.... or possibly go all out and call it a shark.

Basically this is a set of simple instructions for knitting your own, in any size you like.

You will need:

Size 6.5mm needles - both straight and circular on a 1m cable
DK (8ply) yarn. The amount will vary. I would say 3 skeins make a toddler sized one.... (depending on the skein length really) and you would want more for a big one. My adult sized one has approx 1200m of yarn... but it is long.
Zip - optional
Elastic or a length of crocheted yarn (to tie the tail tight)

Step 1: Creating Your Fins

This is much easier on straight needles, 2 fins at a time.

Cast on 6 stitches, twice... so that you have 2 sets of 6 stitches as shown in the images.
Knit 2, purl 2, knit 2. This forms a start of a 2x2 rib for the tail.

This is where the instructions become less precise, and more instinctive.

With the fins on the same needle, recognise that these will end up exactly as you have them. They both need to increase one stitch on the inside of the tail area on every row you knit. This does mean that every row begins on a straight side (counting both fins as one row.... ) Please look at the images to see that increases occur on one side of the fins, not the other.

Every row increase one stitch on the inside.

Maintain a 2x2 rib pattern knitting over the rest of the fin. I did an increase as a knit through the front and the back of the edge that gets increased.

Continue increasing until you have 40 stitches on each fin. Now you can either knit further rows without increasing to make longer fins, or move onto the body of the mermaid tail. The choice is yours.

Step 2: Moving Up the Tail Area

With your fins together, this gets a little interesting.

Inititally, using the fin as a starting point, cast on an additional 40 stitches. Knit across the whole row, both fins. Turn your knitting and cast on another 40 stitches. Purl the whole row.

This gives you a total of 160 stitches... a long section, a fin, another fin, and a long section.

Over the next row you will create a row of holes. Knit 2 together, and then yarn over. Continue this over the whole row.

On the next row, purl all the stitches and the yarn overs.

You can now begin the pattern. The darker blanket in the images has a feather and fan stitch. However, this proved to be a nightmare to accomodate the increases to make the blanket wider as it goes up the leg.

The paler coloured one is a 2 row repeating pattern that works more easily for increasing, so I will put those instructions in here. By all means you can use any pattern you like.

The pattern here is:

Row 1: Knit all the stitches
Row 2: Knit 4, purl 2 together without removing the stitches from the needle, purl the first stitch of the 2 again, then remove both stitches from the needle. Continue this purling motion until the last 4 stitches. Knit 4.

To make this easier to understand, here is a video to help.

Knit 40, place a stitch marker, knit 80, place another marker, knit 40.

Complete row 2 of the pattern.

Continue repeating rows 1 and 2 for 20 rows. You are now ready to increase.
During the next knit row, knit 39, knit 1 front and back (increasing a stitch), pass stitch marker, knit 1 front and back (increasing a stitch) knit 78, knit 1 front and back (increasing a stitch), pass stitch marker, knit 1 front and back (increasing a stitch), knit 39.

Complete row 2

Now.... you can continue to knit 20 rows, then increase 4 stitches (on either side of the stitch marker) until you have the length you desire.

When your blanket is the length you desire, knit 10 rows to complete the top edge. (or alternatively, complete a 2x2 rib over the 10 rows). Cast off.

Step 3: Finishing Up

Firstly, using either a crocheted length of yarn or a length of elastic thread through the holes you knitted near the beginning. Pull these tight. (I used yellow ribbon in the images to make this bit a little clearer.

Stitch up the back seam for half the length of the tail, or insert a zip from the top.

You can now wear your tail... or give it away...