Semi fitted kirtle, back laced. The long-sleeved version shown may be worn alone (over a linen smock) or with a surcoat for warmth. A short-sleeved kirtle should only be worn under a long-sleeved gown or at least with pinned on over-sleeves.
A practical garment, possibly in common use for warmth. Not to be confused with the sideless surcoat which was only ever a robe of state for adults.
In these sizes: