An open heart watch has a small window cut into the dial, usually near the six or twelve o'clock position, through which you can see the balance wheel swinging back and forth. It is the only visible sign that a mechanical watch is alive, and that is the entire point of the design.
What you are actually looking at
The balance wheel is the regulating organ of a mechanical movement. It swings back and forth at a fixed rate, typically several times a second, and each swing releases the escapement to let the gear train advance by one precise step.
Everything else in a mechanical watch is a way of storing energy and passing it along. The balance is the part that decides how fast the watch runs. Opening the dial over it means the wearer sees the mechanism doing the one thing that matters.
Open heart or skeleton?
They are two answers to the same impulse and they are often confused.
An open heart keeps a normal dial and cuts one aperture in it. You get a legible watch with a window onto the escapement.
A skeleton removes as much material as possible from the dial, the bridges and the mainplate, so the whole movement is exposed. It is far more dramatic and considerably harder to read, because there is no solid background behind the hands.
Claude Bernard makes both. The open heart appears in Classic and Aquarider. The full skeleton, with a transparent caseback so the movement is visible from both sides, sits in Proud Heritage.
Is it just decoration?
Functionally, yes. The aperture does not make the watch keep better time.
It does something else, though. A quartz watch and an automatic watch look identical from three feet away. An open heart makes the difference visible without you having to explain it, and it gives a mechanical watch a reason to be looked at rather than merely read. For a first mechanical watch, that visible movement is often what makes the purchase feel worthwhile.
What to check before buying one
Legibility. A large aperture placed under the hands can make certain times harder to read. Look at where the window sits relative to the hour markers.
Finishing around the opening. The edge of the aperture is the detail that separates a considered design from a hole in a dial. Look for a defined, polished or chamfered rim.
What is behind it. Some open hearts reveal a decorated balance bridge; others show a plain one. The view is the feature, so it is worth looking at closely in photographs or at a dealer.
Does it affect servicing?
No. The aperture is in the dial, not the movement, so the service interval is the same as any other mechanical watch: roughly every four to six years, as covered in our piece on servicing. There is no additional dust risk, because the case is sealed as normal.
Common questions
Are open heart watches always automatic?
In practice, effectively always, because there has to be a mechanical balance to display. A quartz watch has no balance wheel and nothing to show.
Is an open heart the same as an exhibition caseback?
No. An exhibition, or transparent, caseback shows the movement from the back of the watch. An open heart shows part of it from the front. Some watches have both.
Why is the window usually at the bottom of the dial?
Because that is where the balance sits in many common movement layouts, and because placing it low interferes least with reading the hands.
Does an open heart weaken the dial?
No. The dial is a structural component only in the sense that it must be flat and stable, and manufacturers account for the aperture when they make it.