Heartbeat-aperture watches
References in the Grail Atlas catalog carrying the heartbeat-aperture complication.
A heartbeat aperture (also called "open-heart" or "balance aperture") cuts a small window in the dial, usually at 6 or 9, exposing the balance wheel in motion. It is a partial skeleton: rather than showing the whole movement, it shows the most kinetically beautiful component, the oscillator that defines a mechanical watch. The format originated in the early 2000s as a way to give entry-tier customers a mechanical-watch signal without going full skeleton.
What to look for
Frederique Constant pioneered the modern open-heart format on the Slimline Heart Beat series and made it a brand signature. Tissot, Hamilton, and Oris all carry heartbeat-aperture references at the entry tier. At higher tiers, the indication is uncommon, most luxury brands either go full skeleton or keep the dial closed. The shopping question is dial composition: a well-placed aperture (Frederique Constant Slimline, Oris Artelier) integrates with the dial layout; a badly-placed one looks like a hole has been drilled. The aperture also affects luminescence, you will lose lume on the index nearest the cutout.
