There are no set rules when writing a knitting pattern that tells you which type of bracket to use for what. It is up to the writer and their personal preference, this is usually influence by who taught them how to write patterns or who they work for. So this is how brackets work in my patterns.
There are two types of brackets, the numbers within, or one to the left, of ether type of bracket, corresponds to the different sizes that the pattern can be knitted.
1st size - 0 - 6 months
2nd size - 6 - 12 months
3rd size - 1 - 2 years 4th size - 2 - 3 years
5th size - 4 - 5 years
Usually, the smallest sizes is given first increasing to the largest available.
Square brackets [ ] represent a variation in the instruction between the sizes that can be knitted, the sizes are separated in my patterns by colons. Sometimes instructions for different sizes can be the same number. Example P4 [5:6:6:5]
1st size - P4
2nd size -P5
3rd size - P6 4th size - P6
5th size - P5
With round brackets ( ) all instructions within the brackets need to be repeated. They are usually followed by a set of square bracket instructions that tell you how many times to repeat what is in the round bracket. Example (P3, p2tog) 6 [6:8:8:10] times
1st size - (P3, p2tog) 6 times
2nd size - (P3, p2tog) 6 times
3rd size - (P3, p2tog) 8 times 4th size - (P3, p2tog) 8 times
5th size - (P3, p2tog) 10 times
Round brackets can even have square brackets with in them. You only need to work the size you are knitting so work each bit step by step, example
P4 [5:6:6:5], p2tog, (p8 [9:7:8:7], p2tog) 6 [6:8:8:10] times, p4 [5:6:6:5]. 63 [71:77:85:91] sts.
If you are knitting the 3rd size you follow the 3rd set of instructions along in each set of brackets, which if wrote out on its own would read: P6, p2tog, (p7, p2tog) 8 times, p6. 77sts.
Some people find it useful to highlight the size they are knitting, it makes it easier to follow at a glace. You can even use different colours for different sizes.