Little Devil's Baby Hat

This is the best baby hat I know. It covers the ears and the forehead, but it leaves out the eyes and eyebrows so that baby can see what's going on around her / him. Garter stitch makes it stretchy.
The original pattern is knit flat and sewn together in the back. It is a traditional German pattern; I think there's someone in almost every German family who knows it or a variation of it. I converted it to be knit in the round (because I don't like seams at the back of a baby's head) and to be adapted to any yarn and head circumference.
I post it in memoriam of my great aunt Ilse who made me one when I was five, and I still wear it.
The model, Teddy Ferdinand, wears a tiny black one from stash yarn.

Update: I am so sorry about the mistakes in the pattern. Revisions are in italics.
If you find any more things, don't hesitate to leave a comment in whatever language. Thank you!

SIZE
made to measure (shown for a head circumference of 15 inches / 38 cm)

1 set double-point needles or 1 quite short circular needle

ABBREVIATIONS
CO - cast on
K - knit
R - round
k2tog - knit two together
prm – put red marker (decrease)
pbm – put blue marker (increase)
kfb - knit into front and back of stitch
ssk - slip two stitches and knit them together

SWATCH
Do swatch, please.
CO 10 stitches.
Work in garter stitch, knitting two together in the beginning of every right side row.
(There is a difference between the swatch and the hat: The swatch is not worked in the round, so, it's normal garter stitch. The hat is worked in the round, with alternating knit and purl rows to result in garter stitch. If your knit and purl stitches are really, really different, you should take that into account for the swatch.)

MATHS (oooh, scary ;-) )
You end up with a triangle. Now, tilt your triangle on the edge so that the cast-on stitches are diagonal on the left hand side and the side with the decreases is vertical on the right hand side.
Pin it out, stretching it the way you want the hat to be on recipient's head.

Measure from the leftmost edge to the other side (across the knitted fabric to the middle of the decrease side). This will give you measure X.

Now, measure Baby's head. If its circumference is, say, 38cm/15in, and your measure X was, say, 4cm/1.5in, you divide 38 by 4 and end up with 9.5 (or 15 by 1.6, also resulting in 9.5. Just make sure you don't mix up inches and cm).
Divide head circumference by swatch diagonal measure (measure X).

If you cast on 10 stitches 9.5 times, it will give you a Little Devil's Baby Hat with a diameter of 38cm/15in. That would be 95 stitches to cast on.
Multiply this calculated stitch number by 10. This will give you the number of stitches to cast on.
You will have to calculate a bit more:
The number of stitches you cast on will be divided into 3 sections. The small one will cover the forehead, the two wider will make the ear flaps. Each ear flap section is twice the forehead section. Therefore, the number of stitches you cast on will be divided by 5.
Divide the number of CO stitches by 5.

In our example, 95 stitches divided by 5 is 19 (giving stitch number A). This number is knit twice for each ear flap and once for the forehead section, so we need to divide it by two. As knitting 9.5 stitches is a bit difficult, we will go for asymmetry and knit 9 for one and 10 for the other part of the forehead section (stitch numbers B and C).
The stitches are separated into sections of 2:2:1:1:2:2

(Security belt: Once you will have worked some rounds, the circumference will seem to be quite big. But notice that the pattern really goes zig-zag around the head. If you're not sure:
Measure along the realistically stretched edge of half an earflap, which gives you measure SB (= security belt ;-) ). You can find out about the head circumference of the hat with this formula: 5 times the radical of (SB squared divided by 2) = head circumference. Or: SB times SB. Divide by two. Take radical. Times 5. This should give the head circumference.)

PATTERN
CO the number of stitches you found out when swatching and doing the maths. Close to knit in the round, being careful not to twist stitches. Mark beginning of the round with blue marker (pbm).
R1 and all uneven rounds: P to end.
R2: In this round, decrease places will be marked with red markers (prm), increase markers with blue markers (pbm).

K A st, prm, K A st, pbm, K B st, prm, K C st, pbm, K A st, prm, K A st (and here comes the blue marker you put at the beginning of the round).
Our example:
K 19 st, prm, K 19 st, pbm, K 9 st, prm, K 10 st, pbm, K 19 st, prm, K 19 st (blue marker already there).

R4 and all even rounds until round 50:
Kfb, K to last two stitches before each red marker. K2tog, ssk.
K to last stitch before each blue marker, increase one by knitting into strand between stitches, k1, increase one by knitting into strand between stitches.
K to last stitch on needle, kfb.



FINISHING
Using kitchener stitch, graft together to form a Y, taking care not to end up with three horns on Baby's head - then, it was the wrong way round ;-)

At one ear flap, CO 6 stitches. Work garter stitch until 6in/15cm. K buttonhole. K half an inch more (one more centimeter), cast off.

Sew button to the other ear flap.