As many things on earth, shawls come in numerous shapes and sizes. The shape variations are a result of strategically placed increases and/or decreases in a mathematical pattern. The five most common shawl shapes are triangular, crescent, semi circular, side-to-side and asymmetrical.  
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.  

The shawl is knit back and forth, where when working on the right side, you work the four increases and on the wrong side no increases are worked.
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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. 

The shawl is knit back and forth, where when working on the right side, you work the four increases and on the wrong side no increases are worked. 

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. ​

Asymmetrical and Side-to-Side shaped shawls are similar in that they both start at a long edge point with very few cast on stitches. Because they are knit from point to point width-wise instead of center out as in the first three shawls, they don’t require a garter tab. 
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. ​