Half-Square Triangle (HST) Quilt Block
The most versatile unit in patchwork — master this and you can make hundreds of different quilt patterns
The half-square triangle is patchwork's most fundamental building block. Two triangles sewn along their long edge create a square unit that can be arranged into pinwheels, flying geese, ocean waves, hourglasses, and hundreds of other patterns.
History & Background
The half-square triangle is one of the oldest patchwork units in existence, documented in American quilts from the earliest years of the republic and in European patchwork going back even further. Colonial quilters knew that triangles waste fabric — cut a square diagonally and you lose the seam allowance to the cut — so early patterns often used triangles to use up scraps that couldn't make a whole square.
By the mid-19th century, HSTs were everywhere: Ocean Waves, Birds in the Air, Bear's Paw, Pinwheel, Flying Geese, and dozens of star variations all depend on HST units. The pattern is so fundamental that it appears in quilting traditions from every continent. In Japan, traditional patchwork forms use the same two-triangle construction; in England, "half-square patches" appear in broderie perse and medallion quilts from the Georgian era.
Modern quilters benefit from accurate rotary cutting and the "2-at-a-time" method — drawing a diagonal line on a square, sewing on both sides, then cutting apart — which produces two perfectly matched HSTs at once. The "8-at-a-time" method, developed in the 1990s, produces eight identical HSTs from two larger squares with a single round of stitching, making it ideal for large projects.
Step-by-Step Instructions
Calculate your cutting size
For a finished HST of X inches, cut squares X + ⅞". For a 3" finished HST, cut 3⅞" squares. The extra ⅞" accounts for seam allowances on both diagonal cuts.
Mark the diagonal
On the wrong side of the lighter fabric square, draw a diagonal line from corner to corner using a fabric marker or chalk pencil. Use a quilting ruler for accuracy — even a slightly off-diagonal line will show up in your finished blocks.
Sew both sides
Layer the marked square right-sides-together with a contrasting square. Sew a scant ¼" seam on both sides of the drawn line. Chain piece multiple pairs to save time.
Cut and press
Cut apart on the drawn diagonal line. Press seams toward the darker fabric (or press open for flat intersections). You now have two HSTs from every pair of squares.
Trim to size
Use a square quilting ruler with a diagonal line to trim each HST to exact size. Line the ruler's diagonal on the seam, trim two sides, rotate, trim the remaining two sides. This step makes the difference between blocks that assemble flat and ones that pucker.
Arrange and assemble
Lay out your HSTs and rotate them to find the pattern you want — pinwheel, arrow, zigzag, hourglass. The same HST can make dozens of different designs purely through arrangement. Sew into rows, then sew rows together, nesting seams.
Tips & Techniques
- Starch your fabric lightly before cutting — it stabilizes the bias edges of HSTs and prevents stretching.
- The 8-at-a-time method: layer two large squares right-sides-together, draw a 2×2 grid of four equal squares on the top fabric, draw one diagonal in each small square, sew ¼" on each side of every diagonal (eight seam lines total), cut on all grid lines, then cut on all drawn diagonals. Eight HSTs from two squares.
- Always trim HSTs after pressing. Even perfectly sewn HSTs can be slightly off-square due to pressing distortion.
- Use a stiletto to hold the point of the triangle at the seam intersection while feeding through the machine — it prevents the point from folding under.
- If your finished HST isn't square, check your ⅞" formula: did you add ⅞", not ⅛"? This is the most common cutting error.
Color & Fabric Selection
HSTs live or die by value contrast. The most dramatic effects come from pairing a light and a dark fabric — the diagonal seam disappears when both fabrics are similar in value and the pattern loses its impact. For scrappy HST quilts, sort fabrics into two value piles (light and dark) rather than by color, and pair any light with any dark.
Variations & Related Patterns
Pinwheel
Four HSTs arranged to spin. One of the most common HST arrangements.
Birds in the Air
Three HSTs forming a triangle unit, creating a flock-like secondary pattern across a quilt.
Ocean Waves
HSTs arranged in rows to create a water-like undulating effect.
Broken Dishes
Four HSTs arranged in a square so the seams create an X — same pieces, different rotation than pinwheel.
Quick Facts
Put it to use
NiftyFifty quilters have been swapping blocks like this one since 1997. Browse our historical archive or join a new swap.
Browse quilt swaps →Related Guides
Flying Geese Quilt Block
BeginnerA classic directional unit with countless uses — from single-row borders to full chevron quilts
Pinwheel Quilt Block
BeginnerFour half-square triangles arranged to spin — one of the most dynamic and beginner-friendly patterns
Quarter-Square Triangle (QST) Quilt Block
IntermediateFour triangles meeting at the center — the essential unit for star points, hourglasses, and pinwheels