AbstractWe 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 first revisit the almost everywhere metric differentiability of Lipschitz
continuous curves. We then show that any blow-up done at a point of metric differentiability and of density
one for the domain of the curve gives a tangent line.
Metric differentiability enjoys a Borel measurability property and this will permit us to use it in the framework
of Lipschitz differentiability spaces.We show that any tangent space of a Lipschitz differentiability 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.
Tangent Lines and Lipschitz Differentiability Spaces