Normed linear spaces which are isometric to order unit spaces

In this paper, we consider the linear direct sum of a real normed linear space with an order unit space and with a base normed space to obtain respectively a new order unit space and a new base normed space. As a consequence, we find that an $$\ell _1$$ ℓ 1 -space may be shown to be an order unit space. (However, not in the natural order.) Dually, an $$\ell _{\infty }$$ ℓ ∞ -space may be shown to be a base normed space. To understand this aberration, we characterize the real normed linear spaces which are isometrically isomorphic to order unit spaces. We prove that other than the classical case of $$\ell _{\infty }$$ ℓ ∞ , $$\ell _1$$ ℓ 1 is also isometrically isomorphic to an order unit space whereas $$\ell _2$$ ℓ 2 is not isometrically isomorphic to any order unit space.