We are concerned with the optimal constants: in the Korn inequality under tangential boundary conditions on bounded sets Ω ⊂ R n , and in the geometric rigidity estimate on the whole R 2 . We prove that the latter constant equals √ 2, and we discuss the relation of the former constants with the optimal Korn's constants under Dirichlet boundary conditions, and in the whole R n , which are well known to equal √ 2. We also discuss the attainability of these constants and the structure of deformations/displacement fields in the optimal sets.L Ω = {A ∈ so(n); ∃a ∈ R n ∀x ∈ ∂Ω (Ax + a) · n(x) = 0} .1991 Mathematics Subject Classification. 74B05.