Sew & Mark Accurate Scant 1/4" Seam Allowances & Trim Triangle Points Using the Prep-Tool

Sew Accurately:

Prep-Tool with 8-page
booklet & 1 Seam Guide

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The Prep-Tool comes with one peel-and-stick, self-adhesive Seam Guide with removable, reusable adhesive. They are 1/32" thick making it easy to align and guide your fabric accuratly before it gets under your needle.

The Prep-Tool also includes an eight page instruction booklet that you can download to view or print.

Replacement 6-Packs of Seam Guides are available separately. Click here to get a pack.

A Scant ¼” is just slightly less than a full ¼” & the
reason it's used for quilt piecing is actually pretty simple.

The thread and fold in a seam have some width. To compensate for that, you sew slightly less than a full ¼”, so your finished seams actually come out measuring a full ¼”.

That way, all the pieces and the finished blocks come out measuring the exact sizes the pattern gives.


Sewing a Scant 1/4" makes the finished sizes of
your pieces, units & blocks come out accurately.

If you sew nine 2 ½" squares together with a consistent Scant ¼” Seam Allowance, your block will measure 6 ½". That's a finished size of 6" with an extra ¼" all around so it can be sewn to other blocks or sashing to make a finished quilt.

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Click here to see how sewing a scant ¼" helps you
get nice pointy points in blocks like these.

The goal is to stick the Seam Guide in front of the presser foot of your sewing machine to help guide your fabric before it goes under the needle.

The Prep-Tool makes it easy to align the Seam Guide so it's a Scant ¼” to the right of the needle.

Guiding your fabric against the edge of the Seam Guide makes it much easier to sew consistently accurate Scant ¼” seam allowances.

Butt the Prep-Tool up against the needle of your sewing machine on the side with the gold lip edges. Peel the paper off the Seam Guide and butt it up against the other side of the Prep-Tool and stick it down in front of your presser foot.

Remove the Prep-Tool and guide your fabric against the Seam Guide for accurate Scant 1/4" seam allowances.

Butt the gold lip edges of the Prep-Tool up to the edge of the fabric, and you should see the line of stitching right along its edge.

If it's inside the edge, you need to adjust the Seam Guide farther away from the needle.

If it's outside the edge, you need to adjust the Seam Guide closer to the needle.

Marking & Measuring Seam Lines for Mitered Corners

The secret to sharp mitered corners, including mitered binding corners is to stop right at the seam line.

The Prep-Tool is a Scant ¼” wide and its raised Lip Edges catch the edge of your fabric, so you’ll always mark the exact stopping point every time.

1) Butt the Lip Edges on the Prep-Tool against the edge of the fabric and mark your stopping point.

2) Sew up to the stopping point and then at a 45º angle into the corner.

3) Fold the end of the binding straight up to form a 45º angle.

4) Keeping the 45º angle underneath, fold the binding back down, and start stitching a scant 1/4" from the edge.

Take the Guesswork out of Set in Seams

When you are sewing just two pieces together, which is the most common thing when piecing, you can sew all the way from one edge to the other.

But many blocks with diamonds and hexagons require a different technique because you'll often be sewing a third piece, set-in, between two others.

For example, each block in a Tumbling blocks quilt is made of three diamonds.

After sewing the first two diamonds together, the third diamond has to be set-in between and sewn to both the first and second diamond.

If you look at the seams from the back when finished, you’ll see that the three seams form a Y.

That’s why set-in seams are also sometime called Y-Seams.

For a Y-Seam or Set-in-Seam, you have to start and stop right at the seam line. If you sew too far, you’ll be reaching for your seam ripper because that will create a pucker that will never press flat.

If you don’t sew far enough, you’ll have a hole where the three seams come together.

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The secret to Set-in-Seams, (3 seams that form a Y),
is to start and stop sewing exactly where the seam lines cross.

Using the Prep-Tool makes it easy:

The Prep-Tool is a Scant ¼” wide and it has raised Lip Edges that catch the edge of your fabric.

Butt the Lip Edges up to the edge of your fabric and you can easily mark the exact point where your seam lines will cross every time.

You can mark the whole seam line

Or, mark just where they cross in the corners.

Start and stop sewing right where
your seam lines cross

Be sure to back stitch or lock stitch
each time you start and stop

Butt the gold lip edges of the Prep-Tool up to the edge of the fabric, and you should see the line of stitching right along its edge.

If it's inside the edge, you need to adjust the Seam Guide farther away from the needle.

If it's outside the edge, you need to adjust the Seam Guide closer to the needle.

Sometimes you need to line up pieces with odd angles, and the edges won't match. If you mark the seam lines with the Prep-Tool, off-set pieces always line up where the seam lines cross.

Prep-Trim Triangle Points Using the Prep-Tool

The lip edges on the Prep-Tool
catch the fabric's edge,
making it easy to accurately line up
& pre-trim points.

If you've worked with Triangles at all, you have definitely trimmed off a few Dog Ears, Bunny Ears or Triangle Points which all mean the same thing.

But most often you've probably trimmed them after you've pressed two Half Square Triangles open into a Triangle Square. When you do that, the Triangle Points are just poking out beyond the edge of the square and it's easy to see where to trim them off.

But there are times when
pre-trimming the points is the way to go.

A good example is if you are sewing a triangle to a square.

The trick is to get the Triangle perfectly centered on the Square.

But because the edges don't match, you have to try to line it up by eye.

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You can look at it this way. These Triangles are the same Finished Size as the Square and if you sew them together it will be the same seam line joining them.

If you look at the seam line on the Square, it is a Scant ¼" from each edge. So if you were to measure a Scant ¼" from the end of the seam line on the Triangle, that is where you would want to trim off the point.

If you used a die cutting machine to cut squares and triangles,

this is how the pieces would come out. Just the finished size with an extra 1/4" all around
and the triangles wouldn't have any points to begin with.

If you accurately trim the points on Half Square Triangles and Quarter Square Triangles, their edges will match with a Square that is the same finished size.

The bottom line to all this is, the edges of Squares and Triangles that are the same Finished Size, should match exactly after you accurately trim the Triangle Points.

Long Side Trim:

Line the Prep-Tool along the Long Side of the Triangle

Short Side Trim:

Line the Prep-Tool along the Short Side of the Triangle

So which trim should you use?

You can usually tell by lining up the triangle to the piece you are trying to sew it to.
You should be able to see which way will make the edges match before you trim the points.

Example: Trimming Triangles for Square-in-a-Square Units:

To make patches out of 3 or more triangles,
you may want to cut your squares into triangles first.

If you cut a Square in half once diagonally, you'll make 2 Half Square Triangles. If you cut a Square in half twice diagonally,
you'll make 4 Quarter square Triangles.

Start by cutting a strip into Squares

Color coded Purple

Cut the Square once diagonally to make 2 Half Square Triangles

Color coded Green

Cut the Square twice diagonally to make 4 Quarter Square Triangles

Color coded Red

A Double Friendship Star has four 3-Part Squares
that are each made of one HST and two QSTs.

Double Friendship Star

Color-Coded Breakdown

3-Part Squares

An Ohio Star has four 4-Part Squares
that are each made of four QSTs.

Ohio Star

Color-Coded Breakdown

4-Part Squares

It Makes it Easier to Line Triangles up When Making 3 or 4 Part Squares.

Use a Short Side Trim on the half square triangles & Long Side Trim on the quarter square triangles.

Short Side Trim:

Line the Prep-Tool along the Long Side of the HSTs

Long Side Trim:

Line the Prep-Tool along the Long Side of the QSTs

They'll line up easier, and sew together easier.

It's also easier to line up and sew the triangles together when you pre-trim the points. The points can get caught in the feed dogs of your sewing machine making it hard to sew in a straight line.

Brave new world units are a combination of:

  • 1 Big Half Square Triangle

  • 2 Small Half Square Triangles that are half the size of the big HST.

  • 1 Square that is the same size as the small HSTs

Use both a short side trim and long side trim
to accurately line the triangles up to the square.

Use a Short Side Trim on one HST

Line it up to the Square and sew right sides together

Use a long Side Trim on the other HST

Now the square and two small HSTs are ready to be sewn to the big HST

You can trim the points on the big HST and the unit you sewed together so their points won't get caught in your feed dogs.

Double trimming makes it easy to line up two diamonds or a diamond and a square.