One of the mysteries of the English language finally explained.
1A wheel that is turned in order to train a cannon or other firearm. Now historical.
2Chiefly North American. In plural A pair of small wheels fitted one on either side of the rear wheel of a bicycle, to add stability when learning to ride.
Mid 19th century (in an earlier sense).
training wheel/ˈtreɪnɪŋ wiːl/
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