Weft Knitted Structures Guide: Jersey, Rib, Purl & Interlock

What Are Weft Knitted Structures?

Weft knitted fabrics are constructed from typically one yarn that is fed into the knitting machine needles in a horizontal direction, a single-yarn logic that tends to shape both fabric behavior and production constraints in weft knitted structures. Each weft thread is fed at right angles to the fabric direction, so loop formation proceeds course-by-course rather than lengthwise along the fabric. The machine can be either circular or flat bed knitting machine, a choice that is rarely neutral and often reflects the required fabric width, end-use, and tolerance for seam placement. The circular knitting machine creates a spiral effect as it produces a fabric in tubular form, an effect that may remain subtle in wear yet become more noticeable after laundering or when prints must align precisely.

Types of Weft Knitted Structures

Depending on the geometrical arrangement of the face and reverse stitches in a knitted fabric (heads, legs, and feet of stitches), there are four basic weft knitted structures or stitches, a convenient classification even though commercial fabrics may complicate it through tuck, miss, or plating effects. These are (a) plain knit structure, (b) rib structure, (c) purl structure, and (d) interlock structure, which are often better read as families than as rigid boxes.

a) Plain Knit Structure

The simplest and most widely used knitted design is the design created by only face loops, and simplicity here does not necessarily imply trivial performance because plain knit is often the baseline against which handle, stretch, and stability are judged in weft knitted structures. The fabric produced from these loops is the single jersey fabric or plain knitted fabric. The single jersey fabric is a single-faced knitted structure with either face or back loop. One side of the fabric consists only of face stitches, and the opposite side consists of back stitches (as shown in Figure), an asymmetry that often explains why single jersey can feel different against the skin depending on which side faces inward.

Figure 2: Plain knit fabric design: (a, b) front side and (c) back side

Jersey fabrics have a technical face, where the wales are prominent, and a technical back, where the courses are prominent, with wales reading as vertical “columns” and courses as horizontal “rows.” The face loops are intermeshed with each other to produce the fabric. A knit stitch is formed by inserting the needle in the back of the loop and pulling a loop of yarn through to form a new loop. The flat knit is also called jersey stitch. In plain knitted stitch, each loop is drawn through other loops to the right side of the fabric. The loops form vertical rows (wales) on the fabric face and crosswise rows (courses) on the back. As such, all the stitches are meshed in one direction, which helps to account for edge curl that is commonly observed when the fabric is cut in weft knitted structures.

Plain knitted fabrics are produced by using one set of needles. The loops are usually V-shaped at the front and semicircular at the back. So the fabric has a different appearance at the face as well as at the back. The face of the fabric is smoother than the back. This fabric is produced either by flat bed knitting machine or by circular knitting machine. The length of the yarn in one loop is the stitch length, and it is uniform throughout the fabric, at least as a target, although in practice it can drift with yarn friction, take-down tension, or machine condition. The amount of elasticity depends upon the stitch length, yarn structure, and characteristics, so two fabrics that “look” similar may still stretch differently if yarn count or twist changes.

The plain knit stitch produces relatively lightweight fabric compared with the thicker fabrics produced by other stitches, a benefit for comfort that can, depending on end-use, reduce opacity and push careful garment engineering. The structure is extensible in both lateral and longitudinal directions, but the lateral extension is twice that of the longitudinal extension. It will be approximately 30–60% stretch across the width and 50–100% across the length, although finishing, fiber type, and stitch density may shift them noticeably. The thickness of the fabric is approximately two times the diameter of the yarn used. The yarn loop pulled in the longitudinal direction would extend by half its length, while when pulled in the lateral direction, it could extend by the entire length. The degree of recovery from stretch depends on the fibers and the construction of the yarn, and recovery in particular may separate “comfortable stretch” from fabrics that bag at knees or elbows over time.

b) Purl Stitch

This stitch is also known as the links/links stitch, a term that can sound old-fashioned but still points to alternating loop presentation across courses in weft knitted structures. It is made on flat bed and circular machines by needles using hooks on both ends. A purl stitch is formed by inserting the needle in the front of the loop and pushing a loop of yarn through to form a new loop, where the direction of needle entry matters because it flips which side of the loop becomes dominant on the fabric surface.

Purl designs are produced by meshing the stitches in neighboring courses in opposite directions by using special latch needles with two needle hooks. The hooks alternately draw loops to the front of the fabric in one course and to the back in the next course. It is a slow and costly technique, with cost arising not only from yarn use but also from machine speed and complexity that can affect production planning. Purl-stitched fabric looks the same on both sides and resembles the reverse of the plain knit. These two stitches are identical but one is reverse of the other. The purl stitch has crosswise stretch and excellent lengthwise stretch; hence, it is widely used in kids and infant wear, a choice that seems reasonable because high extensibility can accommodate movement and growth, though it may also require attention to dimensional stability in washing. The symbolic representation of a purl stitch is shown in Figure 2.

When the fabric is stretched lengthwise, then the face stitches are visible. The fabric shrinks more in the direction of wales, and once it is released, it relaxes to hide the face stitches between the courses. The interlooping of the stitches of neighboring courses in opposite directions results in the courses of a purl knitted structure closing up, a descriptive phrase because the fabric tends to compact vertically and this is felt as thickness and softness in the hand. The structure, therefore, has a large longitudinal extensibility which is largely elastic.

c) Rib Stitch

Rib structure in either a weft knitted fabric or warp knitted fabric is characterized by the appearance of legs on both the sides, so ribs read as alternating ridges and channels and are often selected where cling or recovery is needed in weft knitted structures. This structure can be produced by meshing the stitches in neighboring wales in opposite directions. This is achieved by knitting with two-needle systems which are placed opposite to each other. This design consists of alternating rows of plain and purl stitches constructed so that both the face and back of the fabric look alike. Figure 3 shows the symbolic and diagrammatic representation of the rib structure. This is the simplest rib structure (1 × 1 rib), while wider repeats such as 2 × 2 follow the same logic but can shift hand and stability.

Figure 3: Rib stitch, (a) diagrammatic representation, (1) dial needles and (2) cylinder needles. (b) Symbolic representation

Rib construction is costlier because of the greater amount of yarn needed. It is a heavier structure and expensive fabric, a familiar trade-off in knitting where more yarn and thickness often buy better recovery and warmth. The rib fabric has a vertical cord appearance. Rib fabric can be produced either on a flat rib machine or on a circular rib machine. These fabrics are also known as double jersey or double face fabrics. When the fabric is stretched widthwise, both sides of the fabric show alternately face and reverse stitches in each course. Once the fabric is released, it shrinks in its width, thus hiding the reverse stitches between the face stitches, which is the practical reason ribs grip cuffs, waistbands, and collars.

The more complex and run-resistant fabrics have loops along the length of the fabric, giving a smooth face on both sides. These fabrics do not curl at their edges, and edge behavior is not a minor detail because it can determine whether a cut panel behaves during sewing. Rib structure is elastic and form filling and retains warmth better than the plain fabric. The geometry of the yarn path influences the elastic behavior of the knitted structures, and geometry may be doing as much work here as fiber chemistry in many knitted fabrics. The longitudinal extensibility of the rib structure equals that of a plain knitted structure. The change of direction of the interlooping of the stitches of neighboring wales results in the wales of a rib knitted structure closing up. This results in structures with better elastic properties widthwise. The rib structures in the lateral direction exhibit extensions up to 140%, a value that is plausible in open ribs though it can drop as stitch density increases. As the number of wales in each rib increases, the elasticity decreases as the number of changeovers from reverse to front reduces, so the designer’s choice of rib repeat is, in effect, a choice about stretch distribution.

d) Interlock Stitch

Interlock is the simplest form of double knit fabric, and despite the name it is not merely “two layers,” but a specific intermeshing arrangement that changes bending and stretch within weft knitted structures. Double knit is a circular knit fabric having loops on front as well as back. It is made with a double set of needles to create a double thickness of fabric. Each thickness of the fabric is joined by interlocking stitches. The two layers of loops formed cannot be separated, a functional property because it can affect snagging and wear. Each set of needles knits a 1 × 1 rib with the stitches on one side set right behind the stitches on the other side. Both sides look like the technical face of the single jersey. So the interlock structure is considered as a combination of two rib knitted structures. The reverse stitches of one rib knitted structure are covered by the face stitches of the second rib knitted structure. On both sides of the fabric, therefore, only face stitches are visible, and it is difficult to detect the reverse stitches even when the fabric is stretched widthwise, which helps explain why interlock is often chosen when inside-out appearance must be clean.

The combination of two rib knitted structures in the interlock structure makes the wales or courses in a tight position, and therefore, the interlock fabrics show very poor elastic properties in both directions, though “poor” is context-dependent because reduced stretch may be desirable when shape retention is prioritized over cling. Any material with double knit structure is a two-faced cloth. Either side can be utilized as the right side. This design is usually produced on multifeed circular knitting machines. Double knit fabrics are firmer, are less stretchy, and provide better cover, and cover is not only about thickness because loop compactness also limits see-through, especially under tension. Double knits produce two layers of fabrics, interlooped simultaneously.

In interlock stitching for the production of double knit fabrics, the dial needles and cylinder needles are present exactly opposite to each other. Here, the feeder yarn changes alternately from the dial to the cylinder and vice versa, an alternating path that creates the characteristic interlock geometry. Owing to this, there are two types of needle used in dial as well as in cylinder: long needles and short needles. These long and short needles are present alternately. To control the movement of these needles, double cam system is adopted in both the dial and the cylinder. One cam is used to control the long needles and the other one for short needles. Two feeder yarns are required simultaneously for the interlock fabric. The long dial needle knits with the long cylinder needle at feeder 1, and the short dial needle knits with the short cylinder needle at feeder 2. Figure 4 shows the diagrammatic representation of the interlock stitch with dial and cylinder needles and the long and short needles present in dial and cylinder. The long needles are represented by long lines and the short needles by short lines, and it may help to read this as a timing problem in which needle length and cam track determine which needles rise, knit, and clear at each feed.

Properties of Plain Knit, Rib, and Interlock Structure

The differences in properties of the plain knit structure, rib structure, and interlock structure are present in the Table, a useful summary although actual performance can shift with finishing, yarn choice, and stitch density across weft knitted structures.

Table: Properties of plain knit, rib, and interlock structure

	Plain Structure	Rib Structure	Interlock Structure
Softness	Soft	Soft	Softest
Durability	Excellent	Good	Good
Warmth	Least	More than plain	Highest
Drape	Good	Good	Best
Stability	Unstable, curls when cut, can run from cut edges	Stable, cut ends will lay flat, will not run	Stable, cut ends will lay flat, will not run
Specialty	Durable, less pilling, easy washing	Durable, Soft, elegant drape	Durable, Soft, elegant drape
Laundering	Easy	Requires more care (to keep pilling down)	Requires more care (to keep pilling down)
Uses	Casual dresses, Everyday Tees, shorts, dresses, lining, sleepwear	Tight tees, skirts, dresses, leggings, underwear, collars and cuffs	Elegant tees, shorts, dresses, lining, sleepwear

Conclusion

Weft knitting begins with a simple premise: one yarn forms loops across the fabric width, and those loops underpin weft knitted structures across apparel and related end-uses. From that premise, structure does most of the work. Plain knit offers lightness and easy production, but it also brings edge curl and a tendency to distort. Purl and rib reorganize loop geometry to change stretch and appearance, at the cost of speed, yarn, or machine complexity. Interlock tightens the system further, trading elasticity for cover and a clean face on both sides. The table usefully summarizes these tendencies, though finishing and stitch density can shift outcomes.