The door 'Keep', or 'strking 
plate' is provided with the mortice latch, and is the metal plate that fits to the door casing.
When fitted, the door latch slots into the 'keep' and holds the door in the closed position. Fitting the 'keep' can be a little tricky, as it needs to be very accurately positioned both vertically and horizontally so that the door latch not only enters into it, but is also holds the door in the closed position without the door rattling.
The first step in fitting is to locate the position of the 'keep' horizontally, which is done by closing the door to the point where the latch touches the edge of the door casing, so that the position of the latch can be transferred onto the door casing. These two marks will need to be brought round into the rebated area by using a square as shown here on the right.
The next step is to locate the vertical position of the latch, the point where the back edge of the latch will be on the rebate when the door is closed. The problem here is that when the door is closed, this position cannot be seen, making it difficult to pinpoint. It is possible to find this point be measuring from the back of the latch to the outside face of the door, but an easier method that I use is to mark the sharp front edge of the latch to its height with a pencil so that 
there is a deposit of the pencil lead left on the latch. Then, pull down the handle so that the latch is retrieved and close the door, releasing the handle when the door is in this position, and pull the door slightly backward and forward so that the deposit of lead is transferred onto the rebate of the casing, there by marking the exact closed position of the latch. With this position now transfered, a vertical line can be drawn at this point, which will cross the two horizontal lines on the door casing . Place the 'keep' onto the rebate, centrally positioned to the top and bottom marks of the
latch, and while in this position adjust it until the vertical line corresponds to the inside edge of the keep, and draw a pencil line around it as shown in the drawing on the right.
Remove the keep, and using a sharp chisel recess the area within the pencil lines to the depth equal to the thickness of the 'keep'.
With the door casing recessed, insert the 'keep', and check that the depth is equal to the thickness of the metal, making the required adjustments as required.
While in position, mark the cut-out position through the keep onto the casing, remove the keep and chisel out a hole of this shape to a depth of about 8mm. Now the 'keep' can be refitted, and checked for being the correct depth after the screws have been inserted. If correct, the door should close without being banged shut, and be held in the closed position without rattling.
