I love math ! =]

I was inspired by the blairs grannysquaresampler. I had newer thought of rectangle granny square before and it made me think ... what is the amount of chains necessary to make a rectangle that fits the gap in a blanket?
I was obsessed for a while, I drew some rectangles starting from the outer edge and counted the chains in the beginning. And the more I drew the clearer it got. And that is when the formula was born!

• x - longest edge (I am counting the holes or typical joining points of the typical granny square)
• y - shortest edge
• z - edge length difference z=x-y 
• B - the chains needed for the beginning or middle

B=|4z-1|

btw, the rounds needed to make the rectangular granny square (R) can be caunted too ->
R=y-1

for example:
I need a square that is 4*9
so the x=9, y=4
that means z=9-4=5
B = 4*5-1 = 19
R = 4-1 = 3

this is the scheme for the middle of rectangle and the first round.

The dots are chains, but the small circle is loose chain (it will be needed to form the corners so it will have to hold 9 double crochet stitches!).

To understand the formula better I will show you step by step how I make one of the needed squares for BigBrightBlanket.



1st step
You estimate the size of the gap (or the size of the needed granny square rectangle)

x=6
y=4

2nd step 
Calculations
z = 2
B = |4*z-1| = 4*2-1 = 7
R = y-1 = 4-1 = 3

3rd step
Begin the square :)

I will repeat myself...
the dots are chains, but the small circle is loose chain (it will be needed to form the corners so it will have to hold 9 double crochet stitches!)

4th step
As you crochet the third and last row of the rectangle join it in the gap and woala! :)

Hope you'll find this useful :)

Happy crafting! ;)