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Only the outermost edge of the motif is actually worked in long and short stitches, with the smooth edge at the outside.  After that, all the stitches are the same length, but laid into each other like bricks in a walkway. By splitting the thread of the upper stitch with the needle when making the stitch below it, one can achieve a smooth blending of color.
Stem Stitch/Crewel Outline
One can use almost any stitch in crewel embroidery--there are no 
exclusively crewel stitches, but these are the most commonly used and most characteristic. They are easy to master and with them one can create breathtaking heirloom works.

Let's take a look...
The satin stitch consists of flat stitches laid closely side by side.
To make a padded satin stitch, lay down satin stitches just inside the motif outline in the opposite direction of the desired finished direction, then satin stitch over the satin padding stitches.

One can also use satin stitches to pad beneath other stitches to lend a motif
greater surface relief.
To form a chain, bring needle through from the rear of cloth; pull yarn through; then insert needle from the front into the original hole; pulling yarn through to the back but for a loop on the front side.  Bring through the fabric from the back  and through the loop, and just before pulling loop taut, send needle back through the new entrance hole to form another loop. Work stitches loosely enough for the chain effect to be visible.

The chain stitch is a fast and easy space filler and also provides a greater relief that the crewel outline/stem stitch. 
The buttonhole stitch is worked between parallel lines.  Bring needle through from back to front on the bottom line and stitch back into the top line,  leaving
Seed stitches are very small back stitches intended to create a background pattern of evenly-spaced, but randomly directed, stitches of the same size.

The seed stitch is a good neutral space filler.
Lay down evenly spaced parallel threads extending from one side of the field to be trellised to the other. Do the same in the opposite direction,  forming a checkerboard, or trellis.  Where the threads cross, place a small stitch to hold them firmly to the fabric.  One can also couch these intersections with French knots, cross stitches, or a variety of other
Trellis and couching and the use of the long and short stitch or soft shading,
while not peculiar to crewel, are used in crewel to a far greater extent and effect
than one will find in other surface embroidery styles.
The stem stitch and crewel outline stitch are essentially the same, except in the first, the thread is carried above the needle and in the latter, the thread is carried below.

The stem/crewel outline stitch allows the needleartist to create beautiful curved lines, a smooth outline, or, when stitched in tightly parallel rows, a solid mass. The stitch is worked from left to right.  The stitches twist to form what appears to be a continuous line.
Satin Stitch
Chain Stitch
Buttonhole Stitch
French Knot
Seed Stitch
Trellis and Couching
Long and Short Stitches
(Soft Shading)
thread loose. Bring needle back through on the bottom line, and when stitching into the top line, pass needle through the loop of previous stitch.
Bring the needle through from the back and grasp the thread with the opposite thumb and forefinger, holding it taut. In a circular motion, hook the thread around the needle before passing it back through the fabric. If proper tension is maintained throughout, a firm, bead-like knot will be formed on the right side of the fabric.

Learn to love the French knot and don't be afraid of it. It has many uses--wooly sheep's coats and acorn caps are just two--and it provides a wonderful relief.
decorative stitches, or fill in the checkerboard squares with decorative knots or satin stitches--there are an almost infinite number of possibilities.
Trellis and couching is the great crewel space filler, allowing the needleartist to work very large, but not spend a lifetime covering large areas with densely placed stitches.
The long and short stitch, also known as soft shading, allows for the dramatic gradation of color
for which crewel is known.
Stitch drawings reproduced by permission of Elsa Williams ® Embroidery.
Most of the stitches in the basic crewel stitch vocabulary
will be familiar to needleworkers who have done
any kind of  surface embroidery