@JeremyECrawford Can mc Druid 19 + Wizard 1 scribe 9th level wizard spells into their book and prepare them since they have 9th level slots?
— Aleks (@Twooshorten) March 21, 2016
The PH errata says you must be able to prepare a spell to add it to your spellbook: https://t.co/Uq1bZxWdB4 #DnD https://t.co/qQNxeTHxET
— Jeremy Crawford (@JeremyECrawford) April 22, 2016
That does not answer the question. Can a first level wizard that has a 9th level spell slot, prepare a 9th level spell? If not, why not?
If you followed the errata (and the noted section in the PH about multi-classing, which links off of that), then that is the answer to the question: you determine preparation/scribing, etc, of each class spell separately as if you only had “x” levels in each class. A Cleric-19/Wizard-1 can only copy/prepare 1st level wizard spells because that is the limit of his wizard levels, unlike his cleric levels which allow for up to 9th level cleric spells. (JC was, as he often does/did, attempting to link to errata and/or note where such is specified within the books themselves so as to get people into the habit of actually *using* the books/errata, leaving SA(C) for more proper clarification that hasn’t yet been errata’d or cited elsewhere.)