Learn to Crochet - Groups or Shells
Shells and groups consist of several complete stitches worked into the same place. They can be worked as part of a stitch pattern or as a method of increasing. Groups and shells can be worked in half double crochet, double crochet or longer stitches.
Five double crochet shell: work five double crochet stitches in one stitch (see illustration)
Some shells have a two double crochet stitches, a chain, and then two more double crochet stitches. Some only have three double crochet stitches. The pattern instructions will specify the number and type of stitches for the shell.