Sumptuary Laws are laws or codes enacted to restrict what a government might consider excessive expenditures or improper posturing. Of course, in these United States, we have nothing which would prevent us from buying a dozen jet planes if we wanted to do so and many wealthy folks have collections of interesting cars and boats and all sorts of conspicuous consumption. Although, it is illegal for folks to wear certain uniforms to which they are not entitled: military and law enforcement, for instance. But, in the middle ages England, there were laws which limited, among other things, what kind of fabric one could wear, based on the person’s social status and identity.

You may have known that at that time, unless royal, people could not wear the color purple - it was strictly forbidden and wearing it could result in an accusation of treason, and we all know what happened to folks accused of treason against the crown. (Just one of the outrages surrounding Anne Boleyn’s romance with Henry VIII: she showed up at Court wearing purple before she was married to him, proclaiming herself as royal - which she was not yet - and the King had bought her the fabric: scandalous!)

However, when it came to fashion in merry old England, there were many other similar taboos. 

Even if you could afford it, you could not wear cloth of gold (a fabric with actual gold woven into it) or certain furs, unless you were Royal. Similarly, if you were the wife of a knight, you could wear damask, silk, or taffeta, but you could not if you were of a lower rank.

Everyone could wear linen, but knights were prohibited from using weasel fur in their clothing. Only Royalty and Peers could wear ermine, and one still had to be Noble to wear fox hides.

Only knights and higher social status folks were allowed to wear satin or velvet.

But these laws were not just limitations; there were also compulsory items. Woolen caps had to be worn by males over age 6 on Sundays for anyone lower than a noble; it was mandatory.

Speaking of wool, even the type of wool was restricted to different classes or ranks.

Beyond that, even if you were of the correct rank, you also had to have a certain level of income to be allowed to wear what you wanted.

There were other aspects of Sumptuary Law beyond fabrics and accessories. These could include restrictions on the cut of woman’s gown, gemstones and precious metals, the style of carriages or furniture, the permitted length of a man’s sword, even what food one could eat. 

The reasons given for these rules were stated as minimizing English money going abroad for fabrics and to keep folks from overspending for the good of their souls, but it also allowed people to know each other’s rank and status at a glance.