The primitive shapes are:

 "box" "crow" "curve" "diamond" "dot" "inv" "none" "normal" "tee" "vee"

As for the modifiers:

'l'
Clip the shape, leaving only the part to the left of the edge.
'r'
Clip the shape, leaving only the part to the right of the edge.
'o'
Use an open (non-filled) version of the shape.
Left and right are defined as those directions determined by looking from the edge towards the point where the arrow "touches" the node.

As an example, the arrow shape lteeoldiamond is parsed as 'l' 'tee' 'o' 'l' 'diamond' and corresponds to the shape

Note that the first arrow shape specified occurs closest to the node. Subsequent arrow shapes, if specified, occur further from the node. Also, a shape of none uses space, so, for example, the arrowhead nonenormal is not the same as normal.

Not all syntactically legal combinations of modifiers are meaningful or semantically valid. For example, none of the modifiers make any sense with none. The following table indicates which modifiers are allowed with which shapes.

Modifier box crow curve diamond dot inv none normal tee vee
'l'/'r'XXXX X XXX
'o' X  XXX X

This yields 39 different arrow shapes. The optional second, third, fourth shapes can independently be any of the 39, except the last cannot be none as this would create a redundant shape. Thus, there are 38 · 393 + 38 · 392 + 38 · 39 + 39 = 2,313,441 different combinations.

The following display contains the 39 combinations possible with a single arrow shape. The node attached to the arrow is not drawn but would appear on the right side of the edge.