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I have been doing some reading on great builds for monsters and a lot of them involve yellow locking this ability, red locking that ability, or make sure you DO NOT accidentally yellow lock this ability.

I am now confused and have a nice headache to go along with it.

How the locking mechanics work for monster passive abilities?

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up vote 3 down vote accepted

Those are references to the monster infusion system. Sacrificing a monster crystal to infusion removes the monster from your inventory, but bestows the abilities that monster had into a new one.

Every monster has 10 passive ability slots: when you get a monster crystal, some of these slots are taken up by abilities. If you look at a monster's ability list, you may see a red lock, a yellow lock, or no lock at all next to some of the abilities.

Because I was confusing myself as I was writing this, I'm going to talk about locking from two perspectives separately: what it means for sacrificing a crystal and what it means for infusing a monster.

Sacrificing a monster crystal

  • Red-locked abilities on a crystal are not infusable: you can't pass these off to a new monster during infusion.
  • Unlocked and yellow-locked abilities are infusable and can be transferred during infusion.

Infusing a monster with a crystal

When you infuse a monster that already has its passive ability slots filled, some of those filled slots will be replaced with the abilities from the infusion based on the following rules:

  • Unlocked abilities can be replaced through infusion: lower ranked abilities are replaced first.
  • Red-locked abilities can never be replaced: they essentially cut the number of free slots a monster has for customization. Either the monster starts with them or acquires them through the Crystarium.
  • If you repeatedly infuse a monster with the same ability (like, say, +N to magic), you can "yellow-lock" that ability, which effectively prevents it from being replaced during future infusions.

Essentially, the main difference between red-locking and yellow-locking is that yellow-locked abilities are viral: they can't be replaced,1 but they can be transferred to new monsters through infusion.

Note 1: to a point; there are certain things you can do to remove a yellow-locked ability. See the Yellow Lock Rules in BMSirus's Monster Infusion FAQ for what those are.

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Thanks for the excellent breakdown. I don't know why other forum posts had to explain this in the most confusing way possible. – Adanion Feb 10 '12 at 14:39
Not true, yellow locks can always be removed by infusing the same ability again (infusing HP +X% on top of a yellow locked HP +X% will remove the yellow lock). It is not logical, but that's how it works. – user30692 Aug 10 '12 at 20:36
@brian yes, there are things you can do to remove yellow locks or have them fall off; this answer was meant to be an overview, not a replacement for an in-depth guide. I added a footnote to such a guide that explains those quirks in more detail. – user3389 Aug 10 '12 at 21:08

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