A countably 
-rectifiable
set is a subset 
of 
that, except for a set of 
-dimensional Hausdorff measure
zero, is contained in a countable union of images of Lipschitz
functions 
.
If in addition 
,
the set is called 
-rectifiable.
Thus a one-dimensional rectifiable set is covered, apart from a set of length zero, by countably many images of Lipschitz functions
from subsets of 
,
while a two-dimensional rectifiable set is similarly assembled from Lipschitz surface
patches. An 
-rectifiable
set has an approximate tangent 
-plane at almost every point with respect to 
.
