I need to make my own door jambs from 1x lumber because I have a salvaged antique transom (the whole door is actually restored antique) and couldn't find pre-made ones at Lowes/HD that are long enough. I did notice that the regular sized ones had a notch on each side jamb to set the header piece. I was wondering what the advantage of that is over simply laying the header piece on top of the side pieces. I can probably make notches in my homemade jamb but it would be easier to avoid the trouble and just set it on top of the side jambs.
Can anyone elaborate on the need to make the notches?