Math
 

OK I have not been quilting for a few months and my brain is getting fried with a mental block. If I am creating a square that I want to measure 2 inches finished, and I want it to consist of a light and dark half square triangle, why am I adding 3/8 inch to each square cut. To me it seems like if I have 1/4 inch seam I should add 1/4 inch. Now I understood this before but now I am just coming up blank so please help. Thank you

Here's a tutorial: http://quilting.about.com/od/quickpi...uaretria_2.htm

and it's 7/8" that you need to add.

Because the inside seams are cut on an angle, which requires a smidgen less than the 1/2" you'd need if you were cutting, say, two rectangles from the square.

Fons and Porter also have a ruler that they have demonstrated on their show where you can use strips like jelly rolls and make half square triangle units.

I usually add 1" instead of the 7/8 and trim down. I find it easier then messing around with 1/8" measurements and never have a HST come up short that way. As to why... I don't bother to strain the brain cells trying to figure out why. It works and that is why I do it. I always thought it had something to do with the pythagorean theorem but I could be wrong.

I cut mine larger also and trim them down. Much less brain power involved!

I am going to try this, pieces are always neater when cut larger and trimmed down. If I follow instructions on the pattern I seem to go with those. It is a little confusing to cut different than what is in the directions. Maybe it would be less so if I would rewrite the instructions for cutting. I'm planning on making a Card Trick next and will try the make larger and cut down on it.

Here's a visual to further explain the reason why you use 3/8" instead of 1/2" (which was the original question):
[ATTACH=CONFIG]367480[/ATTACH]

Please excuse my pathetic Paint skills - the drawing is not to scale.

I totally agree about cutting larger and trimming down.
But I think it's always helpful to know the reason(s) why.

this is my method also :)

If it is a plain square that measures 2" finished, then the square is cut at 2 1/2" (1/4 seam allowance on all sides - helps to draw a 2" square then add the 1/4" seam allowance on all sides then measure - I'm a visual person).

If making a half square triangle, you need to add an additional 3/8" for a total of 2 7/8". The diagonal cut takes up the additional 3/8".

I like the tutorial in the link that I posted above it shows how to do this. Also, if you have a problem with accurate seams, just make the squares at 3", put them right sides together, draw a diagonal line on the top one, and stitch 1/4" from the drawn line, the cut on the line, then square to 2 1/2" unfinished size.