The Racah formula for the SU(2) 6-j coefficient is usually considered as a pure combinatorial formula. A physical interpretation is found for this formula and, more generally, for combinatorial formulae of the 3n-j coefficients. Angular momenta are associated with the points of the finite projective geometry PG where triangular conditions appear as collinearities of points. A 3n-j coefficient corresponds to a subset of PG so that some of the angular momenta are hidden for the 3n-j. The combinatorial formula of the 3n-j is interpreted as the summation over these hidden angular momenta of a highly symmetric `full -J symbol'.