What is double?

Answer 1

do you mean dk or double knit yarn ? if so this is basically equal to USA sport yarn. It is non American version and knits at between 5-6 stitches to the inch using a number 5 - 6 needle ........ OR ..... did you mean just using the yarn double as in using two strands at once? if your pattern says use the yarn double then just use two strands at once by pulling from two skeins and holding both threads together when your work .

Answer 2

a is a name of the variable, and double is a datatype.