Question: The book says:

> 
> **Concentration.** Some spells require you to maintain concentration in order to keep
>  their magic active. If you lose concentration, such a spell ends.
> 
> 
> 

So what happens if, for example, you are already concentrating on a spell, and cast Witch Bolt without concentrating. Would you get the initial arc of blue energy, or would it not even go off?

I am asking if you can cast a spell that says concentration without concentrating for an immediate effect. This would be useful to avoid interrupting your existing concentration. In essence, if you are concentrating on *thing A* (doesn't have to be a spell since other things need concentration), can you cast a concentration spell getting an immediate effect without losing concentration on *thing A*?

Since the definition of concentration is to keep the spell active (meaning it is already active) I would think that you would get the initial effect (if it had one) without the need to concentrate on it.

Answer: 
No. Casting a concentration spell immediately breaks any existing concentration.
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One of the rules clarifications in Xanathar's Guide to Everything (pg. 5) addresses this issue directly:

> 
> As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly.
> 
> 
>
A concentration spell requires concentration *all the time*
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You concentrate on a concentration spell from the instant you start to cast it until you stop concentrating. As such any previous concentration spell loses concentration at that instant.