In the case of organized bodies lettres de cachet were issued for the purpose of preventing assembly or to accomplish some other definite act. The provincial estates were convoked in this manner, and it was by a lettre de cachet (in this case, a lettre de jussipri), or by showing in person in a lit de justice, that the king ordered a parlement (a court of justice) to register a law in the teeth of its own refusal to pass it.
The best-known lettres de cachet, however, were penal, by which a subject was sentenced without trial and without an opportunity of defense to imprisonment in a state prison or an ordinary jail, confinement in a convent or a hospital, transportation to the colonies, or expulsion to another part of the realm. The wealthy sometimes bought such lettres to dispose of unwanted individuals.
This meant that when the king intervened directly, he could decide without heeding the laws, and even contrary to the laws. This was an early conception, and in early times the order in question was simply verbal; some letters patent of Henry III of France in 1576 state that François de Montmorency was "prisoner in our castle of the Bastille in Paris by verbal command" of the late king Charles IX.
In the 14th century the principle was introduced that the order should be written, and hence arose the lettre de cachet. The lettre de cachet belonged to the class of lettres closes, as opposed to lettres patentes, which contained the expression of the legal and permanent will of the king, and had to be furnished with the seal of state affixed by the chancellor.
The lettres de cachet, on the contrary, were signed simply by a secretary of state for the king; they bore merely the imprint of the king's privy seal, from which circumstance they were often called, in the 14th and 15th centuries, lettres de petit signet or lettres de petit cachet, and were entirely exempt from the control of the chancellor.
They were also often used by heads of families as a means of correction, for example, for protecting the family honour from the disorderly or criminal conduct of sons. The case of the Marquis de Sade (imprisoned 1777 - 1790 under a lettre de cachet obtained by his wealthy and influential mother-in-law) is a prominent example. Wives, too, took advantage of them to curb the profligacy of husbands and vice versa.
In reality, the secretary of state issued them in a completely arbitrary fashion, and in most cases the king was unaware of their issue. In the 18th century it is certain that the letters were often issued blank, i.e. without containing the name of the person against whom they were directed; the recipient, or mandatary, filled in the name in order to make the letter effective.
It was not until the reign of Louis XVI that a reaction against this abuse became clearly perceptible. At the beginning of that reign Malesherbes during his short ministry endeavoured to infuse some measure of justice into the system, and in March 1784 the baron de Breteuil, a minister of the king's household, addressed a circular to the intendants and the lieutenant of police with a view to preventing the most serious abuses connected with the issue of lettres de cachet.
The Comte de Mirabeau wrote a scathing indictment of lettres de cachet while imprisoned in the dungeon of Vincennes (by lettre de cachet obtained by his father). The treatise was published after his liberation in 1782 under the title Les Lettres de cachet et des prisons d'etat and was widely read throughout Europe.
In Paris, in 1779, the Cour des Aides demanded their suppression, and in March 1788 the parlement of Paris made some exceedingly energetic remonstrances, which are important for the light they throw upon old French public law. The crown, however, did not decide to lay aside this weapon, and in a declaration to the States-General in the royal session of 23 June 1789 (art. 15) it did not renounce it absolutely.
In works set in later time periods, the term Lettres de Cachet is occasionally used metaphorically, primarily applied to unjustified imprisonment of political prisoners, or government abuse of power more generally. For example, in Robert A. Heinlein's science fiction novel Stranger in a Strange Land, Jubal Harshaw uses the term to describe the warrants against himself, Valentine Michael Smith, and Gillian Boardman; the accusation is considered severe enough that the authorities immediately back off to avoid appearing tyrannical.