Epic levels and incomplete casting classes

[Heliomance]

How do they work? If I'm level 20, but only have 14 spellcaster levels due to multiclassing, what happens when I tick over to level 21? Can I continue advancing my casting like normal until I have 20 spellcasting levels? Can I pick up a brand new spellcasting class after level 20 and gain the normal casting for that class? It's all very confusing.

And how do casting-progressing PrCs work? If I'm a Wizard 15/Incantatrix 5 (yes I know, you'd never do that, whatever), what am I allowed to pick at level 21? What does that do to my casting?

[TheNivMizzet]

After level 20 you can finish your spellcasting up the the cap of 20, at level 26. At which point you no longer add spell slots. But, you can still improve your caster level, and there are feats for adding higher level spell slots, which can be used for metamagic.
For the Incantatrix/Wizard, at level 21 you would become CL 21, and would again, need the feats to continue spell slots.

[Curmudgeon]

BAB and saves are the only things that are automatically cut off after level 20. Other than those, you can continue any class you've got for as many levels as are in the class. It's only after you maximize the spellcasting of a regular class, and you're also at least level 21, that you can use Improved Spell Capacity (http://www.d20srd.org/srd/epic/feats.htm#improvedSpellCapacity) to get past the pre-Epic spell level limits for you class(es).