Most triangular, crescent and semi-circular shawls are knit “top down” or starting with a small number of stitches and increasing out, however some are knit “bottom up” where a large number of stitches are cast on and strategically placed decreases are used to make the shape of the shawl. Asymmetrical and side-to-side shawls are knit point to point width-wise.
The most iconic shawl shape is a top down triangular shawl. This shawl requires four increases every other row to maintain its shape. The increases are placed two at the outer edges and two down the spine of the shawl. These placements of increases make obvious triangular segments that increase in size.
The most common method of starting a triangular shawl is called a garter tab. This is where a small rectangle is knit and becomes the top border edge of the shawl by picking up and knitting stitches around three of its sides.
Crescent shaped shawls are similar to triangular shawls in that they start with a garter tab but are different because there are more increases placed only at the beginning and end of the rows on both sides of the work. Crescent shaped shawls generally have two increases on either end of the row on the right side of the knitting and one increase on either end of the wrong side of the row. These six increases produce rapid growth in fabric along the edges and the crescent shape.
Semi circular shawls also start with a garter tab and their increase placement is more geometric, giving the shawl a straight top edge but curved outer edge. In a semi circular shawl there are a four increases evenly placed across the knitting, two at the edges and two that radiate evenly from a single point. These increases, created by yarn overs, in the picture below are clearly visible. Modification of the type of increase will make them less visible.
The side-to-side shawl construction requires one increase every other row. When you’ve reached the desired shawl depth the increase changes to a decrease.
The example below has one increase on the right side of the knitting at the leading edge but on the wrong side of the work has an decrease at the beginning of the row and an increase at the end of the row. These two increases and one decrease over two rows create an increasing triangular shape with a bias.
Below are Ravelry links to some good examples of the different shawl shapes that I’ve favorited.
Crescent Shawls - http://ravel.me/K-Rae/cssos
Asymmetrical Shawls - http://ravel.me/K-Rae/assos
Semi-Circular Shawls - http://ravel.me/K-Rae/sssos
Side to Side Shawls - http://ravel.me/K-Rae/stsssos
Triangular Shawls - http://ravel.me/K-Rae/tssos
If you’re interested in further exploring shapes and construction of shawls I recommend this free online course by Aroha Knits. http://www.arohaknits.com/5-shawls-5-days-challenge-sign-in/ Or check the local LYSs for shawl classes.