Under a short-sleeved jersey...
Here it is under Gore's ALP-X 3.0 jersey; the sleeves don't show, nor does it hang below the jersey's bottom hem line.
The overall fit isn't as sloppy as it looks here (the jersey, ALP-X 3.0, has a relaxed-fit...so unlike Giordana, which of course is made to be aero dynamic; but it lets tons of air flow through the mesh back and front sleeve area). The base layer also has a mesh back, so that together you are pretty well ventilated air/heat flow from/on back area (even with a pack). But on the down side, if the wind changes direction, watch out! Be sure just to take along a good jacket, just in case!
Overall, this base layer does just what it is designed to do; and very light weight.
A sleeveless version is also available, for under singlet's.
There's a long-sleeved zip mock Windstopper base, too...a definite need for cooler/colder blustery rides; and the zip helps to vent heat a little, too.
I like the utilitarian-ess of the almost wrap around (but not really all the way around the front) back/side pockets of the jersey...you can actually access them on board.