Can someone explain the 7/8 rule to me?
 

I'm just starting to dip my toe into triangles and wanted to try drawing out a block and making it (there's only so many 8" blocks online). I'm new enough to sewing that my triangles and seams are imperfect even if the measurements are right and I don't even feel like I should know what good triangles should look like mid-piecing.

I understand that adding 7/8 to triangles pre-piecing is "correct" but I have no clue where the extra 3/8 goes and how an almost 2" triangle becomes 1", especially if my seams have an 1/8 margin of error (because they probably do).

Can anyone explain why this works and how to tell if you're on track during the piecing process?

The extra 7/8" is added to the measurement of the original square that you cut. So, to make a 2" finished half-square triangle, you would cut two squares 2 7/8". You then cut them on the diagonal, and you will have the triangles to sew together to make a finished 2" triangle. To make the same size square, you would cut a 2 1/2" square for a finished 2" square. If your seams have a 1/8" error margin, then it is better to go up a full inch, sew the half-square triangles and then cut them to size after sewing. Especially when you are just starting to learn. For a quarter-square triangle block, you will cut the squares an additional 1 1/4" (or 3 1/4" for a 2" finished square) and then cut on both diagonals. Again, if you are just starting and not really great at precision piecing, go up 1 1/2" and trim after sewing the entire square.

Why does this work? Because of the way the seam allowance works in a triangle. Because the seam allowance ends up sticking out a little bit and making the dog ear that is then cut off, the triangles have to have a side that is longer than the finished side.

I hope that makes sense.

Here is a good tutorial from Connecting Threads:

http://www.connectingthreads.com/tut...ngles__D4.html

We on the board can help- have no fear!!!! there seems to be 2 issues here 1) your seams should be 1/4" not 1/8" +/-. test the seam width by cutting and sewing 2- 2 1/2" strips togetyher, press. The resulting piece should be 4 1/2". If they are not then keep adjusting needle or moving fabric on the base until you get the perfect 1/4" seam. Be sure to put a mark on the place where you get the 1/4".

2) the 7/8 measurement refers to how much bigger you cut your square when making Half Square Triangles (HST). for exampole- to make a finshed 4" HST you cut 2 pieces of fabric 4 7/8", place right sides together draw a line on the diaginal, sew 1/4" on either side of this line and then CUT on the line. You cut on the line after you sew the squares together. I am not sure what ypou are referring to when you talk about triangles. Most patterns have you make HST using the aboue method. Lots of ladies make their Squares 1" bigger and then cut down to needed size. Why 7/8" because that is what the quilt police say!!!!!( and mathamatically that is correct).

Ok. If I make my triangle bigger because I occasionally get my seams a little off and cut it down how do know how far down to cut. It would seem like I'd just get a bigger triangle? I just want to understand what should be happening mathematically because I know my sewing is imperfect and I will likely need to adjust if I'm ever going to get them "right" but I need to know how I should be adjusting, if that makes sense.

So here's where I really started questioning the 7/8 rule. I have. A small 1" square that I want to put in a square as a center motif. That means the the outside triangles have a 1" hypotinous and 3/4 inch square size finished. Would I really use a 1 5/8 sqare to make the outside triangles? Or is there a point where the rule breaks down? It's important to me to learn how to design my own blocks.

OK, now you are dealing with a totally different calculation. There are many different types of triangles. You add 7/8" to the finished size of your target finished patch for Half Square Triangles (HST) only.
I'm not 100% sure what you are asking, but I think you want to put 4 triangles around a center square. This is called a square in square (SIS).
One thing to determine is the finished size of the inside square. Are you cutting a 1.5" square to wind up with a 1" finished size, or are you actually starting out with a 1 inch cut square, in which case the finished size is 0.5"
Here's some good info:
http://www.equilters.com/library/PFP...in-square.html
Here's a calculator:
http://www.quiltersparadiseesc.com/C...Calculator.php

It will be 1" finished. I've calculated all of my finished sizes and I just need to figure out my cutting sizes. Your calculator links seem to make alot of sense and coincide with my measurements so a big thank you paper princess. I think there's more I don't fully comprehend about quilt math, but this should get me started again.

I had no intention of learning why the 7/8". I just made it simple and bought a ruler that only had 7/8" measurements. No need to add the extra to the measurement and I always have a solid line to measure from.

https://www.youtube.com/watch?v=Eg3VTy6NAXg

As an aside, Quilter's Cache has a whole section of 8" blocks, there's at least a couple hundred. Scroll down past the small list of 7" blocks.

http://www.quilterscache.com/BlocksbySizeB.html

Sounds like you could use Deb Tucker's Square Squared ruler. Here's a link to her video:

http://www.studio180design.net/videos/?id=3

Her philosophy is to cut your pieces slightly over-sized and trim down to the exact size needed for your project. I just finished a t-shirt quilt and used her Tucker Trimmer ruler to make over 250 HST's. No matter how careful I am, my blocks always turn out a tad bit small when I only add an 7/8" to my blocks. It's more time-consuming, but I would much rather trim 256 blocks down to the exact 2.5" needed, then have 65 blocks that are only 2.25" when I'm done. Each of her rulers come with an instruction sheet that tells you exactly how big to cut all of your pieces to trim down to your desired size. The square squared ruler sounds like what you're trying to do. I am very happy with each of my Deb Tucker rulers.