Petticoats
Petticoat was a term first used during the early 15th century, when it was actually used to describe a gentleman’s short coat or undershirt. Later on in the century, it was adopted for women’s dress and worn beneath a gown as an underskirt — only a small glimpse of the lacy under layer would peak through. Since then, the petticoat has evolved from plain to ornate, and is now used to add body to loose fitting skirts. Today, a petticoat hem describes the ornate lace or embellished hem that is added underneath a skirt — more as an extra flourish, than as an extra layer.
Pleats
Pleats are a fold or doubling of fabric that is pressed, ironed or creased into place. (Pleats that are sewn into place are called tucks). There are many varieties, but the side and box pleat are the most common, although they can be accordion, cartridge, circular, curtain, draped, fluted, Fortuny or French. They can be inserte, as well as inverted. Pleats add an even greater fullness to a shape (particularly the skirt). Think Marilyn Monroe over the air vent in ‘The 7 Year Itch’.