The drawing shows the tool I use to align the edge of a window apron with the casing above the sill. I take a scrap of 1x (in this case, a piece of baseboard stock) about 10 in. long. With a jigsaw, I cut a U-shape out of the middle of one edge. To align the apron, I press the tool against the jamb casing with the U going around the overhang of the sill. I slide the apron over until it meets the bottom of the tool, and then I drive a quick nail to hold it in position.
Billy McGraw, South Yarmouth, MA
Edited and Illustrated by Charles Miller
From Fine Homebuilding #221
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I've always heard this type of tool referred to as a "preacher" because it never lies.
I always wondered about that name- your explanation never occurred to me.
Actually, the preacher I'm used to uses the face of the u-shaped board and is generally used to mark cut length for trim- see this link; scroll to the second page for pix
https://books.google.com/books?id=TOwQE77E1WMC&pg=PA48&lpg=PA48&dq=preacher,+finish+carpentry&source=bl&ots=rwHGKg0uku&sig=Wq9gbZk5O6acTp5rE-3fsGXMmJ0&hl=en&sa=X&ved=2ahUKEwjkq_C9qM7fAhXHFzQIHW-
HCkcQ6AEwA3oECAcQAQ#v=onepage&q=preacher%2C%20finish%20carpentry&f=false
An alignment block is also essential for installing the jamb casing - to get a consistent 3/16" reveal against the extension jamb - which is often not 'square' because it is mounted on rough framing.
Another tip: When you install the extension jambs, DO NOT quickly tack them against the framing! Framing lumber is often warped - and even a 1/16" 'bow in the middle of a window will create a nearly unsolvable problem with the casing trim. This is a classic problem: the extension jambs are weak in the same direction that the casing trim is strong. So the extension jambs need to be perfectly flat. Check the framing to see if it is warped. If not, no problem. If the framing is warped, trim screws are very helpful with the extension jamb - because you can adjust the distance, unlike a quick 'whack' with a pin nailer.
I hope these details are helpful.
Mark
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