Fig. 4.

Illustration of the reduction of arbitrary joint structures and of right-tight structures, Procedure (a), and of tight structures, Procedure (b). In the bottom row the symbols for the 10 distinct types of structural components are listed: A, B maximal secondary structure segments R[i, j], S[r, s]; C arbitrary joint structure J_1,N;1,M; D right-tight structures J^RT_i,j;r,s; E double-tight structure J^DT_i,j;r,s; F tight structure of type ▽, △ or □; G type □ tight structure J^□_i,j;r,s; H type ▽ tight structure J^▽_i,j;r,s; J type △ tight structure J^△_i,j;r,s; K hybrid structure J^Hy_i,j;h,ℓ; L substructure of a hybrid J^h_i,j;h,ℓ such that R_iS_j and R_hS_ℓ are exterior arcs and J^h_i,j;h,ℓ itself is not a hybrid since it is not maximal; M isolated segment R[i, j] or S[h, ℓ].