Hi. I am studying Multivariable Calculus and found simply connected regions difficult to understand. Why is an xy plane with the positive x-axis removed a simply connected region while an xy plane with the entire x axis removed is not simply connected? In the latter case, as x-axis is not defined, we shouldn't even be able to draw any curve crossing axis... So this leaves me with two separated planes in which everywhere is defined and differentiable. Then why is it still not simply connected? How about the former case? I am confused.