Provisional Cast Ons (PCO) are very handy for:
- finishing both the top and bottom edges of a knit piece in the same manner
- beginning a piece in the middle and knitting outward in both directions
- add ribbings to a sweater – don’t work the ribbing at the beginning, begin with the main pattern using a PCO. Then, pick up the PCO sts and add the ribbing later.
- Especially great because you can easily lengthen cuffs and bottom ribbings for growing kids.
- Adjust the final sleeve / body length of a garment after the main piece is finished.
I like the crochet method linked above since it’s easy to undo later – you just unzip the crochet chain as you catch the PCO stitches. Always use a smooth yarn in a contrasting colour to make the PCO easy to undo.