An example of geometric construction

A PG(3,2) gives a BIBD(15,35,7,3,1). Using homogeneous coordinates x and y to denote the points, a plane is represented by the equation α'x=0. The correlation may be defined by x'Py=0: to the point x corresponds the set of points y such that (x'P)y = 0 (and vice versa). Here, the matrix adopted is:

0 1 0 0
1 0 0 0
0 0 0 1
0 0 1 0
and this gives the following correspondence between points and planes, where lower case letters a,b, etc denote the points of the planes, having the same coordinates as the upper case letters. (For example, a = (0001), d = (0100), both satisfy the equation (0 0 1 0)x = 0 involving the point A which is also (0001), and the matrix P above, and so both belong to the plane α corresponding to A.)
Point Coords Plane
A (0001) a,d,e,h,i,l,m
B (0010) b,d,f,h,j,l,n
C (0011) c,d,g,h,k,l,o
D (0100) a,b,c,d,e,f,g
E (0101) a,d,e,j,k,n,o
F (0110) b,d,f,i,k,m,o
G (0111) c,d,g,i,j,m,n
H (1000) a,b,c,h,i,j,k
I (1001) a,f,g,h,i,n,o
J (1010) b,e,g,h,j,m,o
K (1011) c,e,f,h,k,m,n
L (1100) a,b,c,l,m,n,o
M (1101) a,f,g,j,k,l,m
N (1110) b,e,g,i,k,l,n
O (1111) c,e,f,i,j,l,o

By constructing the lines of the geometry from the points, and the corresponding lines from the intersections of the corresponding planes, the following partial design is obtained (in resolved form):

BHJ,bhj ALM,alm FIO,fio EKN,ekn CDG,cdg
AHI,ahi FKM,fkm CLO,clo BEG,djn DJN,beg
CKH,ckh ADE,ade GIN,gin BMO,fjl FJL,bmo
BLN,bln GHO,cij AJK,ehm CEF,dko DIM,afg
BDF,bdf EHM,ajk CIJ,gho GKL,cmn ANO,eil
GJM,gjm FHN,bik ABC,dhl DKO,cef EIL,ano
EJO,ejo DHL,abc BIK,fhn CMN,gkl AFG,dim

All that remains is to match the points within the lines: in the original paper this was done by trial-and-error, but some improvement to that can be made.