Consider boundary conditions. A slip, for example, could be
considered to be a turn so uncoordinated that there is no turn -- what
you see out the front window just goes by sideways.

Don

Well, no, it's just a slip - the plane is flying sideways through the
sky. The turn needle won't show any turn, but the ball will be at
full deflection.

OTOH, try a "flat" turn sometime. Full rudder, enough opposite
aileron to keep the wings level. With enough patience, you can turn
this way, although stopping the turn on a specific heading becomes a
bit of a math problem (think about it)!

Watch out for spinning out of this one, though...

Kirk