My preprint "Projective lines as groupoids with projection structure" is now available at arXiv. A projection structure on a groupoid consists in giving, for each pair (A,B) of distinct objects in it, a bijection between the set hom(A,B), and the set of objects distinct from A and B. Groupoids with such structure occur naturally in projective geometry over a field. - The present note investigates when, conversely, an abstractly given groupoid with projection structure comes about from a field, constructed out of the groupoid. http://arxiv.org/abs/1310.5357 Anders [For admin and other information see: http://www.mta.ca/~cat-dist/ ]