This Audemars Piguet Royal Oak reference 15100ST.OO.0789ST.01 is a limited edition of 450 pieces made for the Foundation Time for the Trees. In the 15100 it takes the Royal Oak into a restrained 36mm format, a size that keeps the line's sharp geometry intact without asking for the broader stance of the Jumbo. Its automatic movement suits that thinner profile, and for a collector who wants the Royal Oak in a more compact register, this is the reference that makes the case cleanly. The stainless steel case measures 36mm across and 8.2mm thick, slim enough to preserve the model's crisp proportions. A blue dial keeps the watch close to one of the Royal Oak's defining signatures, while the date remains its only complication. Tied to a specific charitable edition, it shows how comfortably the Royal Oak absorbs commemorative production when the underlying design is already this settled.