Tangent Lines and Lipschitz Differentiability Spaces

We study the existence of tangent lines, i.e. subsets of the tangent space isometric to the real line, in tangent spaces of metric spaces. We rst revisit the almost everywhere metric di erentiability of Lipschitz continuous curves. We then show that any blow-up done at a point of metric di erentiability and of density one for the domain of the curve gives a tangent line. Metric di erentiability enjoys a Borel measurability property and this will permit us to use it in the framework of Lipschitz di erentiability spaces. We show that any tangent space of a Lipschitz di erentiability space contains at least n distinct tangent lines, obtained as the blow-up of n Lipschitz curves, where n is the dimension of the local measurable chart. Under additional assumptions on the space, such as curvature lower bounds, these n distinct tangent lines span an n-dimensional part of the tangent space.