Drafting the Basic Trouser Block and Sleeve Block

Why the Trouser Block and Sleeve Block Matter

The basic block—sometimes called a sloper—is the foundation from which every pattern variation is cut. A trouser block built accurately to your client's body measurements will grade, rotate, and style without accumulating error. The sleeve block, once trued to the armhole of a fitted bodice block, becomes the starting point for every sleeve silhouette in your portfolio. Both credentials you are working toward—the CTDA tailoring credential and the Marine Fabricators Association MFC—expect you to produce pattern work that fits without repeated correction, so precision at the block stage saves time at every later stage.

Tools and Materials

Work on dot-and-cross pattern paper placed on a smooth table at a comfortable standing height. You will use a straight ruler, an L-square or set square, a hip curve (French curve), a flexible tape measure, a tracing wheel, and sharp pencils. Keep a clear work surface: loose rulers sliding under your arm while you draw cause inaccurate lines and can fall, creating a trip-and-cut hazard. When cutting paper with scissors or a rotary cutter, keep the opposite hand well behind the cutting edge and use a cutting mat.

Key Measurements Before You Begin

For the trouser block, take and record:

• Waist circumference — at the natural waist
• Hip circumference — at the fullest point
• Crotch depth — seated measurement from natural waist to chair seat
• Crotch length — total crotch seam length, front waist to back waist through the legs (taken with tape standing on edge)
• Inseam length — inside leg from crotch point to desired hem
• Outseam length — outside leg from natural waist to desired hem

For the sleeve block, you need:

• Armhole circumference — walked from the finished bodice armhole with tape on edge
• Sleeve length — shoulder point to wrist
• Bicep circumference — at fullest point of the upper arm

Record every measurement twice and compare before drafting.

Drafting the Trouser Front Block

Using the Armstrong formula sequence, begin at the top-left corner of your paper. Drop a vertical line equal to the crotch depth measurement. This is the crotch depth line. From the top of this line, extend a horizontal waist line. Drop another vertical to establish the knee line at approximately half the inseam length, and a final horizontal at the full inseam length for the hem.

The crotch extension on the trouser front is typically one-quarter of the hip measurement divided by four, then reduced by a small formula constant—check your Armstrong chapter for the exact constant used in this edition, as editions vary. Draw the crotch extension outward from the side-seam line at the crotch depth level, then swing the crotch curve upward using your hip curve tool. The curve must be smooth—no flat spots or kinks—because a poorly drawn crotch curve translates directly into discomfort and poor hang on the body.

True the inseam and outseam by laying the ruler from the crotch point and hem point in sequence, then blend into the waist curve and the hip curve with the French curve. Add 1/2 inch ease at the hip (ease is already included in most Armstrong formula drafts, so check before adding more).

Drafting the Trouser Back Block

The back block is wider and longer in the crotch than the front. The back crotch extension is approximately twice the front extension. The back also carries a back rise angle: the center-back seam leans outward at the waist to accommodate the seat curve. This angle is typically about 1 inch out from vertical at the waist for a standard figure, but the Armstrong sequence gives you a formula approach rather than an estimated number.

Draft the seat curve on the back using the same hip curve tool, making the curve deeper and longer than the front. True both the inseam and outseam again. Check that the front and back inseam lengths match exactly from crotch point to hem—walk them together with tape on edge.

Drafting the Sleeve Block

Draw a vertical grain line. From the top, mark the sleeve cap height (also called cap height or crown height). Cap height is calculated as a proportion of the armhole depth from the bodice block; the Armstrong formula gives a specific ratio. A taller cap height gives more shape and is appropriate for tailored garments; a shallower cap suits sports or casual wear.

From the cap height point, extend a horizontal bicep line. The distance from center to each side of the bicep line is one-half of the bicep circumference plus ease. For woven tailoring fabrics, sleeve cap ease typically falls in the range of 1 to 1.5 inches; this ease allows the fabric to be eased smoothly around the sleeve head without puckering.

Draw the front and back cap curves using the hip curve, shaping them so the front cap is slightly flatter and the back cap has a bit more curve. Walk the sleeve cap against the armhole: stand the flexible tape on edge and measure the front bodice armhole separately from the back, then compare to the front and back halves of the sleeve cap. The sleeve cap should exceed the armhole by the target ease amount, distributed roughly 40% to the front and 60% to the back—confirm this split and write it on your block label.

Labeling and Trueing

Every finished block must be labeled with: piece name, grain-line direction, measurement set used, date, and your name. Mark all notches. Trueing—checking that adjacent seams match in length where they will be sewn together—is not optional. Walk every seam pair before you call a block complete. Errors caught on paper cost seconds; errors caught on the body cost fabric and hours.

Safety Reminders

Keep rulers flat on the table when not in use. Use a cutting mat and safety ruler when using a rotary cutter. Good posture at the drafting table protects your back during long sessions—adjust table height so your elbows are at roughly 90 degrees.