Every auction catalogue for older wine describes fill levels in careful terminology. That gap between the surface of the wine and the base of the cork is the single most useful condition signal available without opening the bottle.
Wine leaves the bottle in two ways
A cork is porous and slightly compressible, so a very small quantity of wine escapes through and around it over decades. This is normal and unavoidable in any bottle of age.
Heat accelerates it dramatically, because warming wine expands and pushes past the cork, and the volume does not return when the bottle cools again.
A low fill in a young bottle therefore indicates a storage problem rather than the passage of time. The distinction between the two causes is what a buyer is trying to read.
The terminology is standardised by shape
Bordeaux bottles have defined shoulders, so fill levels are described by position against them, running from into-neck downwards through upper shoulder and mid shoulder.
Burgundy bottles slope continuously and have no shoulder, so their fills are measured in centimetres from the base of the cork instead.
The two systems are not interchangeable, and a centimetre measurement conveys nothing about a shouldered bottle without knowing where the shoulder sits.
What each level implies about handling
A fill into the neck on a bottle of real age suggests either exceptional storage or a bottle that has been recorked at the estate.
Upper shoulder is normal for a wine of several decades and generally considered sound. Mid shoulder invites caution and prices accordingly.
Below mid shoulder, the risk of oxidation rises steeply, and such bottles sell as curiosities rather than as drinking propositions.
Recorking changes the reading
Some estates offer to inspect, top up and recork old bottles, which restores the fill and resets the visible evidence.
A recorked bottle is usually marked as such, and the topping wine is normally a younger vintage of the same wine.
Opinion divides on whether this improves or compromises a bottle, since the intervention preserves the wine while altering it. Buyers price recorked lots differently depending on which view they hold.
Reading fill alongside everything else
Fill level is most informative when read with capsule condition, label staining and the cork's visible position in the neck.
Seepage staining under the capsule confirms heat exposure, and a cork pushed slightly proud of the rim points the same way.
A consistent story across all these signs is more reliable than any single one, which is why serious catalogues describe every bottle in a lot individually.