How to Sew the Mosaic #21 Quilt Block - a Traditional Block with a Modern Look

By: Elaine Huff

Are you ready to dive into a project that marries tradition with a touch of modern flair? Today, I'm thrilled to walk you through the Mosaic #21 quilt block - a delightfully simple block that only uses half square triangles. This tutorial is designed to guide beginners and seasoned quilters alike in creating a stunning piece that feels both classic and contemporary. So grab your favorite fabrics and let's get stitching on this eye-catching design that promises to be a fantastic addition to your quilting repertoire.

mosaic #21 quilt block

So where does the #21 come from? That's the name the Ladies Art Company gave it. It probably was the 21st in a line of mosaic blocks that they featured. However, Nancy Cabot called this block Mosaic #6. Which ever name it's still a pretty block!

Fabric Requirements for a 12" Finished Mosaic #21 Quilt Block:

Light/White: 8 - 4" squares
Medium/Purple: 3 - 4" squares
Dark/Burgundy: 3 - 4" squares
Dark/Blue: 2 - 4" squares

Sewing Directions:

Half Square Triangle Units:

Step 1:

Using three Light/White 4" squares and three Medium/Purple 4" squares, make Half Square Triangle units as shown. Here are the steps corresponding to the images in the picture below by going from left to right, top to bottom.

Match up a Light/White square and a Medium/Purple square right sides together. Mark a diagonal line from corner to corner on the back of the Light/White square and sew 1/4" (or a little less) on each side of the line.
Cut along the diagonal line. You will have 2 half-square triangle patches.
Open up the patch and press the seam allowance toward the Light/White fabric.
Square up/trim the units to 3.5" square.

Step 2:

Repeat for a total of six White/Purple Half Square Triangle units.

You can find a detailed tutorial for sewing Half Square Triangles (HST) at https://fabric406.com/blogs/fabric406-blog/how-to-sew-a-basic-half-square-triangle-block

Step 3:

Repeat Steps 1 and 2 only substituting the Medium/Burgundy for the Medium/Purple fabric. Press the seam allowances toward the Burgundy fabric.

Step 4:

Repeat Steps 1 and 2 only using two Light/White 4" squares and two Dark/Blue 4" squares for a total of four White/Blue Half Square Triangle units. Press the seam allowances toward the Dark/Blue fabric.

Putting It All Together:

Step 5:

Lay out all the Half Square Triangle units as shown in the picture below.

Step 6:

Sew the units into four rows as shown below.

Step 7:

Press the seam allowance in opposite directions from row to row. You can see in the picture below that I've pressed rows 1 and 3 toward the left and rows 2 and 4 are pressed toward the right.

Step 8:

Sew the four rows together.

Step 9:

Press the seam allowances whichever direction you wish. I've pressed them all in the same direction. You're finished!

Layout Ideas:

Here's your basic 4 x 4 layout with similar colors to the tutorial:

This is the same layout but with narrow sashing and cornerstones added.

Here's an example of the same block but with an alternating block that has a similar diamond design but in different colors.

This is another example of using an alternating block. This time I used a chain block and a monochromatic color scheme of pinks and burgundys (burgundies?).

This version uses a large Hourglass block for the alternating block.

This is an on-point layout in Christmas colors and using a simple chain for the alternating block and plain background fabric for the setting triangles.

Here's another on-point layout with narrow sashing and cornerstones in a complementary green.

In the example below, I used the Mosaic #21 as the alternating block with a simple star block.

Conclusion:

While the Mosaic #21 is a quick and easy block to sew, it does have multiple design possibilities! I enjoyed sewing the block and had fun playing with the different layouts. Win-win!

I hope you have enjoyed this tutorial. If you like it and want to see more, just click HERE to sign up for my newsletter and receive a free PDF quilt pattern called "Chained Weathervane".

Happy Quilting!

Elaine

P.S. Just looking for the basic blocks? Check out this link: https://fabric406.com/blogs/fabric406-blog/tagged/basic-blocks-tutorials

P.P.S. Check out my Etsy shop at https://www.etsy.com/shop/fabric406