Wikipedia
The Telemann-Werke-Verzeichnis (Telemann Works Catalogue), abbreviated TWV, is the numbering system identifying compositions by Georg Philipp Telemann, published by Martin Ruhnke.
The prefix TWV is generally followed by a genre number, a letter indicating the key (in some cases), and a work number. The genre number indicates the general type or medium of the work. A major key is in upper case, a minor key in lower case. The second number is the work's number within the genre. For example, Telemann's Concerto polonois in B flat major for strings and basso continuo is TWV 43:B3. His Orchestral suite in D major is TWV 55:D18, and his Overture in G minor is TWV 55:g4.
Vocal works were catalogued in a similar way by Werner Menke in the Telemann-Vokalwerke-Verzeichnis (Telemann Vocal Works Catalogue), abbreviated TVWV. For example, Telemann's Johannes-Passion is TVWV 5:42. His Times of the Day cantata is TVWV 20:39.