I would say 12 feet.
Or you could say that for the endpoint of the spike, y=12.0.
Any point in our 3D scene can be assigned a representation by measuring its distance along the three axes. For these notes (and for many computer graphics books), these three distances are put into a column matrix as follows.
You are probably accustomed to putting the three coordinates into a triple such as (x, y, z). But it will turn out that for computer graphics it is more convenient to put them into a column, as above.
For example, the coordinates of the corner of the building that lies along the X axis are (about):
What are the coordinates of the corner point of the white lattice along the top edge (labeled P0)?