Backpack

Backpack=

Piggyback=

Krampus' Sack=

"That's my rucksack."

- Woodie

The Backpack is a Survival Item that serves to expand the character's inventory. It requires 4 Cut Grass, 4 Twigs to craft and Science Machine to prototype. The Backpack itself cannot be placed in inventory or Chest and only one may be equipped at a time.

When a Backpack is created it automatically equips itself in the Chest Slot if it is available. Wearing the Backpack provides 8 additional inventory slots. This item uses the same equipment slot as Armor and Vests, which means wearing one requires sacrificing the benefits of the other.

Although Backpacks cannot be placed in inventory or in Chests, they can be placed on the ground and used for storage. Any item clicked on a Backpack which is on the ground will give the 'Store' option allowing the pack to be filled without equipping it.

Piggyback
The Piggyback is a type of Backpack which requires 4 Pig Skins, 6 Silk, 2 Ropes and an Alchemy Engine to prototype. It is a larger Backpack which has 12 inventory slots but reduces walking speed by 20% while equipped. It has 4 more slots than a Backpack, but 2 fewer slots than the Krampus Sack. It cannot be placed in the inventory or in Chests.

As of the Reign of Giants update, A Feet of Strength, in RoG-enabled worlds the piggyback and it's contents are waterproof and will not cause sanity drain while equipped. The piggyback does not give the player any rain resistance, and used in conjunction with an umbrella will not protect the rest of the player's items from wetness. The movement speed penalty was also reduced from -20% to -10% in this update. Used with the walking cane, the penalty is negated and the player's speed increased to 115%.

Krampus' Sack
The Krampus' Sack is a wearable storage item, like Backpack. It is an extremely rare drop from Krampus, with only 1% of drop chance. It has 14 inventory slots, which is 6 more than a Backpack and 2 more than a Piggyback. It cannot be placed in the player's inventory or in Chests, and it can't be lit on fire.

Even though the importance of only two extra slots may be minimal, the Krampus Sack doesn't slow the player down like the Piggyback does, so it is better in every way. However, with a lowly 1% drop rate and a significant time investment to spawn a Krampus, it should be said that during any given game it's possible that the player might spend more time trying to get the item than would be saved by not walking slower with the Piggyback. It may be better to ultimately treat the Krampus Sack as an unexpected bonus and/or something to only strive for in one's spare time.

Trivia

 * The Backpack was added in the The Birds and the Bees &amp; Dogs That Want to Kill You update.
 * The Piggyback was added in the Doorway to Adventure update.
 * The Krampus Sack was added in the Naughty and Nice update. Before the Underground update, the Krampus Sack didn't appear as an icon on the map.
 * When Krampus wears the Krampus Sack, there are no chains surrounding the Sack, whilst the item obtained from killing Krampus has two chains draped across it.
 * The player can negate the Piggyback's slowing effect by equipping a Walking Cane.