Painted with squirrel-hair brushes on vellum or parchment by monks, nuns, and urban craftspeople, these bizarre images populate the margins of the most prized books from the Middle Ages.

Their illustrations often tell a second story as rich as the text itself.

Some images appear in many different illuminated manuscripts, and often reinforce the religious content of the books they decorated.

For example, a porcupine picking up fruit on its spines could represent the devil stealing the fruits of faith, or Christ taking up the sins of mankind.

Medieval lore stated that a hunter could only capture a unicorn when it lay its horn in the lap of a virgin, so a unicorn could symbolize either sexual temptation or Christ being captured by his enemies.

Rabbits, meanwhile, could represent human's lustful natures and could redeem themselves through attempts to make sacred music despite their failings.

All of these references would have been familiar to medieval Europeans from other art forms and oral tradition, though some have grown more mysterious over the centuries.

Today, no one can say for sure what the common motif of a knight fighting a snail means— or why the knight so often appears to be losing.

The snail might be a symbol of the inevitability of death, which defeats even the strongest knights.

Or it could represent humility, and a knight's need to vanquish his own pride.

Many illuminated manuscripts were copies of religious or classical texts, and the bookmakers incorporated their own ideas and opinions in illustrations.

The butt tuba, for example, was likely shorthand to express disapproval with – or add an ironic spin to – the action in the text.

Illuminations could also be used to make subversive political commentary.

The text of the "Smithfield Decretals" details the Church's laws and punishments for lawbreakers.

But the margins show a fox being hanged by geese, a possible allusion to the common people turning on their powerful oppressors.

In the "Chronica Majora," Matthew Paris summarized a scandal of his day, in which the Welsh prince Griffin plummeted to his death from the tower of London.

Some believed the prince fell, Paris wrote, while others thought he was pushed.

He added his own take in the margins, which show the prince falling to his death while trying to escape on a rope made of bed-sheets.

Some margins told stories of a more personal nature.

"The Luttrell Psalter," a book of psalms and prayers commissioned by Sir Geoffrey Luttrell, shows a young woman having her hair done, while a young man catches a bird in a net.

The shaved patch on his head is growing out, indicating that he is a clergyman neglecting his duties.

This alludes to a family scandal where a young cleric ran away with Sir Geoffrey's daughter Elizabeth.

The family's personal spiritual advisor likely painted it into the book to remind his clients of their failings and encourage their spiritual development.

Some artists even painted themselves into the manuscripts.

The opening image of Christine de Pisan's collected works shows de Pisan presenting the book to the Queen of France.

The queen was so impressed by de Pisan's previous work that she commissioned her own copy.

Such royal patronage enabled her to establish her own publishing house in Paris.

The tradition of illuminated manuscripts lasted for over a thousand years.

The books were created by individuals or teams for uses as wide-ranging as private prayer aids, service books in churches, textbooks, and protective talismans to take into battle.

Across all this variation, those tricky little drawings in the margins are a unique window into the minds of medieval artists.