Kingfish wrote:



There is no fixed boundary between approach and center airspace.


There is always a fixed boundary, laterally and vertically. One thing
you can be sure of with ATC is that only one controller owns the
airspace at a time.



Here in CT, once you climb past 15k
or so you get handed off to Center.



The handoff occurred much earlier and means that the controller working
you now can let you enter the next controllers airspace.



I've flown VFR up to Maine below
10k and talked to Center - it's probably based on letters of agreement
over who owns what airspace and at what altitude. When at or above 18k
(Class A airspace) you're talking to Center no matter where you are,
although there may be exceptions to this rule in some places

Controllers can always coordinate with each other for certain things.
For example if one of the local rich guys needs to fly up to FL350 over
my airport to check on the new engine but he'll stay within 15 or 20
miles of the airport I'll just call center on the landline and tell him
I'll keep him. That means the center controller has to separate all his
planes from my guy.