Positional scanning was introduced independently by Furka et al. and Pinilla et al. The method is based on the synthesis and testing of series of sublibraries. in which a certain sequence position is occupied by the same amino acid. The figure shows the nine sublibraries (B1-D3) of a full peptide trimer library (A) made from three amino acids. In sublibraries there is a position which is occupied by the same amino acid in all components. In the synthesis of a sublibrary the support is not divided and only one amino acid is coupled to the whole sample. As a result, one position is really occupied by the same amino acid in all components. For example, in the B2 sublibrary position 2 is occupied by the "yellow" amino acid in all the nine components. If in a screening test this sublibrary gives positive answer it means that position 2 in the active peptide is also occupied by the "yellow" amino acid. The amino acid sequence can be determined by testing all the nine (or sometime less) sublibraries.