Two new ternary platinum borides, YPt B and YbPt B , were obtained by argon-arc melting of the elements followed by annealing at 780 °C (750 °C). The structures of these compounds combine the fragments of CaB - and AuCu -type structures space group 3̅ ; = 1.15, = 4.0550(4) Å and = 1.34, = 4.0449(2) Å for YPt B and YbPt B , respectively; single-crystal X-ray diffraction. Two possible variants of B/Pt ordering (space group 4/ ) were created via a group-subgroup approach targeting the derived stoichiometry. The architecture of the type- YPt B structure model ( ' = , ' = , ' = ) combines the 4.8 boron nets alternating with the layers of Y and Pt; the type- YPt B structure model ( ' = 2 , ' = 2 , ' = ) exhibits columns of linked B truncated cubes filled with Y running along the axis. The striking features of both structural models are B Pt octahedra. The structural similarities with hitherto reported structures (YB C , M Ni B , MNi B , and ErNiB ) were drawn supporting the verity of these models. A chemical bonding analysis for type- and type- YPt B based on electron localization function distribution revealed a two-center interaction forming the 4.8 boron nets for type- YPt B and a covalent bonding within B Pt octahedra as well as a two-center interaction for B-B intraoctahedral bonds for type- YPt B . Analysis of Bader charges revealed the cationic character of the yttrium atoms. The interactions for nondistorted areas of the structures agree well with the bonding picture calculated for constituent building structures, YB and YPt . Electronic structure calculations predict YPt B to be a metal with the density of states of around ( ) = 1 states eV f.u. . The exploration of the Y-Pt-B system in the relevant concentration range elucidated the homogeneity field of YPt B (0.90 ≤ ≤ 1.40) and revealed the existence of three more ternary phases at 780 °C: YPt B (space group 6 22), YPt B (space group 4 ), and YPt B (space group 2/ ).