Corridors

All the stones on the second row are alive.  The short lines coming out of the stones on the second row denote these stones are alive.

There are two types of corridors. 
A blocked corridor is closed at one side of the first row with the stone of the same color as the corridor. The opponent may invade from the other side.
An unblocked corridor is open to invasion on both sides. The opponent may invade from the both sides.

Here is an example.

The left is blocked corridor of length i , and 
the right is unblocked corridor of length i.