I've been crocheting with 2 different brands of yarn recently and both are double knit, AU 8 ply.
Both are quality brands, but differ in thickness, which forces me to use a different size hook.
Here I am working on the crown of a baby hat
I used a 4 mm hook on the left and 3.5mm on the right
The yarn on the left is slightly thicker that the yarn on the right, so I adjusted the hook to suit the yarn
Yarn hook and ply does make a big difference when making a wearable item from specific instructions.
Take one of my own patterns for example.
Simple Crochet Sweater Size 2
Free Crochet Pattern
I used a 4mm hook and dk yarn (the thicker dk) making the sweater below.
Debbie Sealock Hughes writes
Here's
your pattern for the free crochet pattern size 2 simple crochet pattern. I had a thicker
yarn so I used a size I hook and it worked beautifully and will fit my 8
year old granddaughter. It is a beautiful pattern and very easy. Took
me about 7 hours to complete. Thanks for the pattern!
With some experience on hand, we can up size and down size a pattern by switching hook sizes and yarn ply's.
I know many do with the patterns I write and it's fun to see!