 The background of traditional contemporary Western dress codes as fixed in 20th century relied on several steps of replacement of preexisting formal wear, while in turn increasing the formality levels of the previously less formal alternatives.Thus was the case with the ceasing of the justacorps, extensively worn from the 1660s until the 1790s, followed by the same fate of the 18th century frock (not to be confused with frock "coat"), in turn followed by the frock coat.Before the modern system of "formal", "semi-formal", and "informal" was consolidated in the 20th century, the terms were looser.