Beta sheetsEach beta sheet is labelled using the single letter assigned to it by the secondary structure assignment program. The number of strands in the sheet is recorded as well as the type of sheet (antiparallel, parallel or mixed). If the sheet forms a closed beta-barrel this is also mentioned. Finally the table gives the topology of the sheet using the nomenclature of Richardson (1981). Each connection between strands is defined by a number which represents the number of strands it crosses over in the sheet and in which direction, with an 'X' added for crossover connections. Thus a beta hairpin will have a +1 or -1 connectivity and two parallel hydrogen bonded strands will have +1X or -1X connectivity.