Wrist-worn watches existed in the nineteenth century as ladies' jewellery, but the decisive shift is military: an officer needing the time with both hands occupied benefits from a watch already oriented on the wrist.
Early wristwatches are visibly converted pocket watches. Small pendant-watch movements sit in cases with soldered wire lugs, enamel dials carry luminous numerals, and the crown remains where a pendant crown would be.
Relocation created new engineering problems. The watch now faced shock, dust, moisture and constant positional change, which pushed development of shock protection, gasketed and screw-back cases, and stronger balance staffs.
Movement architecture adapted next. Purpose-designed small calibres replaced adapted pendant movements, and the centre-seconds layout displaced the subsidiary seconds inherited from pocket practice.
For collectors this transition is the most legible in horology: an early wristwatch usually declares its pocket-watch ancestry in its movement, its dial layout and its case construction at the same time.