My questions are these:
When I am knitting from the bottom up, and have completed 18 rounds, how is the next round a "Hem Rnd" and why is it purled?
*I suspect the first 18rows are the brim, which you flip up/fold, thus the purl row is where the fold is when the hat is worn. Or, even if not folded, the "hem" separates the brim from the crown.
Also, the crown is done from a chart which is repeated all the way around, starting from the center of the chart going from left to right. On the right hand edge of the chart there is what is called "Center stitch for double decreases".
*Often times the chart is laid out so that you can see the image in its entirety. This is because the image is centered on the object (hat, sweater front), but you start knitting from the sides. You need to start your rounds/rows where directed per the directions.
What does this mean? Is this the center or starting point instead of the middle of the chart?
*You will need to read the directions to find out, or post a pattern link for me to assist you further here. I suspect, this is the starting point for your rounds, and the center back of the hat. The double decreases likely occur here rather than on the more visible front/image of your hat.
What is a double decrease?
*A decrease that decreases 2 sts.
The pattern only indicates k2tog. Is this a double decrease?
Some rounds call for "sl 1, K2tog, psso". Is this what is called a double decrease?