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Join the primary vest yarn to main body piece of the vest using a slip stitch. Find the end of front panel A, then skip three unworked double crochet in the main body piece. Join the yarn onto the main body stitch after that. Chain four. Skip the next double crochet in the main body piece, then double crochet once into the double crochet after that. Chain one, skip the next double crochet, and double crochet once into double crochet after that. Repeat 27 times. For medium vests, repeat 27 times. For large and extra-large vests, repeat 35 times. Turn the work and leave the remainder of the main body stitches unworked. Do not work any of the back panel rows into this untouched section of the main body. For the second row of the back panel, follow the same steps used to create the third row of the main body piece. Work one beginning half-shell into the first double crochet of the row. Skip the next three double crochet, single crochet once into the double crochet after that, and skip the following three double crochet after that. Shell stitch once into the next double crochet. Skip the following three double crochet, single crochet once into the double crochet after that, and skip the next three double crochet after that. Repeat this substep pattern across the rest of the row. Work one half-shell into the last double crochet of the row, then turn the work. Chain one, then single crochet once into the first double crochet. Chain one and single crochet once into the next chain-one space; repeat once more. Chain one and skip the next two chain-one spaces. Single crochet into the next chain-one space and chain one; repeat three more times. Skip the next two chain-one spaces. Repeat this entire substep sequence until you reach the last half-shell of the previous row. Single crochet in the next chain-one space and chain one; repeat once more. Single crochet once into the last double crochet, then turn the work. For rows 5 through 15 of the back panel, you'll need to repeat steps used while working previous rows. For row five of the back panel, repeat row three of the body. For row six of the back panel, repeat row seven of the body. For rows seven through eleven of the back panel, repeat rows two through six of the back panel. For rows 12 through 15 of the back panel, repeat rows two through five of the back panel. Cut the yarn, leaving 3 inches (7.6 cm) of excess. Pull this tail through the loop on your hook to fasten off the yarn. Weave the excess tail into the inner stitches of the vest to hide it.
Join the yarn. Double crochet across the first back panel row. Repeat the third body row. Shell and half-shell in the third row. Single crochet across the fourth row. Work a series of repeat rows down the rest of the back panel. Fasten off the yarn.