Pyramidal Crystal Face Numbers

Pyramidal crystal face numbers are an extension of those for plates and columns. Top and bottom hexagonal basal faces, if they exist, are numbered 1 and 2. The middle prism faces are 3 to 8. The top pyramidal faces are 13 to 18 and the lower pyramid faces are 23 to 28
	A 9° odd radius halo is made by 3-26 rays i.e. rays entering the middle prism face 3 and leaving through lower pyramid face 26. The wedge angle is 28° compared with the 60° paths 3-5 giving the more common 22° halo. 4-27 or 5-28 or 7-24 rays are equivalent to 3-26 but the path description is always simplified to the lowest face number where possible.