I could not make perfect half triangles until I purchased this ruler. Elenor Burns uses this ruler in her "Underground Railroad" quilt book. I bought the ruler at my LQS, however here is a link.

here is the link to it http://www.quiltknit.com/RU295-TRIAN...AY_p_2519.html

slashed line on sewed line according to size of block
Name:  Attachment-56135.jpe
Views: 75
Size:  50.6 KB

trim edges
Name:  Attachment-56136.jpe
Views: 81
Size:  47.7 KB

finished perfect size square
Name:  Attachment-56137.jpe
Views: 75
Size:  35.5 KB