Medio vs. mitad: Expressing the beginning and of a period or space

Question

I understand your explanations perfectly, but I was surprised to learn that it was correct to use "mitad" and "medio" interchangeably when discussing physical space.

It seems to me that way back when I was first learning those concepts, I was told that medio meant middle and and mitad meant half, and that it was an error to confuse the two. Is this a case of one of those "errors so common among native speakers that now it's not wrong anymore" ? Or was I taught incorrectly to begin with?

Thanks for the insight!

Accepted Answer

Hola A
It depends on the context. Sometimes medio and mitad both mean "half", for example, when you are talking about quantity:

Me he comido medio bocadillo.
Me he comido la mitad del bocadillo. 
I have eaten half [of] my sandwich.

They are interchangeable here.
They are also interchangeable in the cases expressed in this lesson.
But there are specific contexts where we use one or the other and they are not interchangeable. Here you can see some of the different uses of medio and mitad to have a rough reference.
Saludos