It is used where it is desired to reduce the height occupied by the upper timber.

Joists are notched on to wall plates.
_No.21.A checked joint or double notch_, Fig.

265, is made by cutting out notches from both the timbers so as to engage each other.
It is used where a single notch would weaken one member too much.
_No.22.A cogged or corked or caulked joint_, Fig.

265, is made by cutting out only parts of the notch on the lower piece, leaving a "cog" uncut.

From the upper piece a notch is cut only wide enough to receive the cog.