Leather armor in the Middle Ages

In the first chapter of my fantasy novel Roots Run Deep, a team of goblinkin are preparing for a raid on a human city.
 
Like anyone who lived on the Reservation, Kip went armed at all times. Not that she had much. Her tattered leather jerkin gave scant protection, and for weapons she carried a flint knife and a tfaa, a traditional goblin fighting stick. A balanced, two foot-long rod carved from ironwood, the tfaa didn’t look like much, but in skilled hands it could disarm and cripple a swordsman. Prenta had gotten rid of her showy clothes and dressed in a more practical leather jerkin similar to Kip’s.

These impoverished goblinkin are wearing leather armor because they can't afford anything better and their human rulers forbid them from bearing metal weapons and armor. (This ban doesn't last, but that comes later in the story. . .) Leather armor is a staple of fantasy fiction and roleplaying games, yet many people don't realize just how common it was in medieval warfare. Even knights wore it.

Leather armor goes back to ancient times and continued in use through the Middle Ages and into the Renaissance. This picture shows the shoulder and upper arm portions of an elegant suit of leather dating to the Italian Renaissance, courtesy of the Schola Forum. As you can see, it looks much like regular metal armor, and many historians believe that it was worn as much as or even more than metal armor. A suit of plate was hot and heavy, so on the march or during a friendly tournament the knight may choose to weather cooler leather. The rank and file would also be fitted with leather and perhaps some portions of metal armor for vulnerable places such as the head and chest.

While leather was much cheaper, it provided pretty good protection. Regular soft leather wasn't much help, but combined with quilted padding provided some protection, especially against blunt weapons such as maces.

More effective was cuir bouilli--boiled leather. If you soak leather in water and then place it in boiling water, it becomes elastic and pliable. It soon begins to shrink, thicken, and harden. As it's hardening, the leather is hammered onto shaped blocks to create breastplates, greaves, vambraces, and anything else. A full suit of armor could be made in this way.

The 14th century French chronicler Jean Froissart claimed that it was "leather that no iron can pierce" and while that may be overstating the case, leather armor certainly gave good protection. Games such as D&D probably undervalue its effectiveness. Modern experiments show that the average sword blow wouldn't get through, although a good English longbow would make short work of a leather breastplate! This basic article (PDF) explains the technique. Also check out this thread from Schola Forum for some more insights and pictures.

The picture below from The Historians' History of the World shows some improvements on leather armor, with overlapping metal scales, discs, and rings. These were all cheaper yet pretty effective alternatives to full plate and variants of these were used from ancient times into the Renaissance.