Tetraforce

'Tetraforce' is a name given to the Triforce by people that believe that the Triforce has four pieces instead of three. Belief in the Tetraforce's existance was sparked by people who noticed a fourth triangle on Link's Hylian Shield and jumped to the conclusion that it was a fourth piece of the Triforce that had been separated from the others, hence the name 'Tetraforce'.

This theory is widely regarded as being foolish, though there are still some people in the community that support it. It is speculated by some that the missing "piece" represents time, and some supporters have used Zelda's mention of a Goddess of Time in Majora's Mask as evidence. However, detracters point out that it is only mentioned in passing while she bids goodbye to Link; there's also some evidence to suggest that Nayru (Goddess) and a goddess of time are the same being.

Other theorists claim the 'Goddess' represented by the Desert Colossus could be viewed as evidence for a fourth peice. This Goddess would explain the existance of the Gerudo, as while Farore created life to uphold Nayru's law, this Evil Goddess may have created life that would break the law.

Another possibility is that the fourth inner piece represents hope; this theory comes from the oracle games. Since there is the oracles of seasons, ages, and secrets each coresponding to a piece of the Triforce(seasons=power,ages=wisdom, and secrets=courage) and eventually Zelda comes along claiming to be the oracle of hope, possibly meaning that she is correspinding to the fourth piece of the tri(tetra)force. It is also possible that Zelda had actually always been a goddess of hope sent to Hyrule to be continuously reborn as the Princess of Hyrule.

Some people believe Zelda is the 4th goddess, hence her pirate reincarnation's name, Tetra.

It is believed that when one has the complete Tetraforce, it is impossible to control its power. People use Fierce Deity Link from Majora's Mask as an example. It is believed that in a past incarnation in the Zelda timeline, Link had found the complete Tetraforce and become immortal. He could not control his power, and he caused much destruction, so the Goddesses created the Song of Healing, which was used to seal the Fierce Deity inside a mask. It is thought that Fierce Deity Link may have been evil in the past since the child wearing Majora's Mask on the Moon tells you that you will be playing the bad guy when he hands you the Fierce Deity Mask.