At 18th level, a Wizard gains the feature [Spell Mastery](https://www.dndbeyond.com/classes/wizard#SpellMastery-413). It reads:

> 
> At 18th level, you have achieved such mastery over certain spells that you can cast them at will. Choose a 1st-level wizard spell and a 2nd-level wizard spell that are in your spellbook. You can cast those spells at their lowest level without expending a spell slot when you have them prepared. If you want to cast either spell at a higher level, you must expend a spell slot as normal.
> 
> 
> 

Early in Merlin's career, they learned the *shield* spell. It's in their spellbook and they have it prepared almost every day. At some point in their life, they realized that the magic was in them all along and gained a level of Sorcerer. As such, they learned *shield* as a Sorcerer to free up a Wizard Prepared Spell slot for something else.

When Merlin reaches level 19 (18 Wizard/1. Sorcerer) they get Spell Mastery.

Can they still use the spell prepared as a Sorcerer for the Wizard feature?
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The checkboxes I see are:

* [X] A 1st-level Wizard spell
* [X] A spell that is in your spellbook
* [?] You have [it] prepared

To my reading, it doesn't need to be prepared AS A WIZARD, just prepared. The question is, Sorcerers don't "prepare" spells, they "know" spells. But the PHB [lumps those terms together](https://www.dndbeyond.com/sources/basic-rules/spellcasting#KnownandPreparedSpells). Is there a mechanical/rules difference and can Merlin keep the spell as a Sorcerer and use it with the Spell Mastery feature?