It is called "partial seaming" ... the first strip is left unsewn towards the end, since it is actually longer than the piece it is being attached to. The reason it is longer is because on the fourth piece, you are going to attach (second, third, and fourth pieces, actually) just as though you were doing a log cabin ... once the fourth strip is attached, you will notice your first piece needs its seam stitched down to make the square ... it is a great technique!
I have lots of strips pre-cut, and I thought (since I love scrappy), it wouldn't be anything to cut a square, (hold on to the original piece) stitch the four patches and then go back through the pieces I cut the squares off of and get the strips I needed to complete the blocks ... this is a very FINE pattern for those of us who are "scrappers"! ... it would be sort of cool, if we could get the figures for each sized strip ... for example:
just sort of figuring on email here - - if I used my 1.5" strips ... those turn in to a three inch four patch ... and, and my strips to go on the four patch would have to measure that 3" + the extra for the next strip - - that would be 4.5" ... is that correct?
so, if I wanted to know how big a scrap to keep for my "scrappy retro" ... it would be 1.5" + 4.5" ... so I would need to save six inch strips for that size ...
do we need the math for other sizes? Have I confused you enough? Did my thinking online help you figure out a formula? <wave>