# Manifold locally looks like a open set but not as a euclidean space?

I am reading about manifolds from the book by Millman. He says that manifolds locally looks like a open set, but there is no canonical way to make M look like a Euclidean Space, and so we can't define tangent vectors in general sense. My doubt is even in $\mathbb{R^n}$, we check differentiablity only in an open set near a point rather than the whole space. So, why are we concerned about the manifold not looking like Euclidean space? Also explain difference between M is locally an open set and M is locally an Euclidean space. This paragraph is from the book i am reading.

• What do you mean by "M is locally an open set"? As for your question "why are we concerned..." my answer would be that having the entire $M$ an open subset of $R^n$ would eliminate the need for transition maps. Thus: We are not concerned. – Moishe Kohan Mar 13 '14 at 20:10