Twin Quartz is the name for two similar thermocompensation technologies employed by Seiko in the late 1970s and early 1980s, resulting in astonishing accuracy as high as 5 seconds per year in certain high-accuracy quartz movements.

All quartz movements vary in rate according to temperature, and the challenge of thermocompensation has long presented challenges. Ever since the first Beta project chronometers, various techniques have been employed.

In the 1970s, Seiko added a second quartz crystal to their leading movements, resulting in various Twin Quartz movement families. Suwa Seikosha was first, introducing their Cal. 99 family in August 1978. Their approach used a second quartz crystal not for timing but to detect changes in temperature and tune the timing of the primary crystal accordingly. This resulted in accuracy as good as 5 seconds per year in the Seiko Superior Cal. 9983. Other members of the Cal. 99 family achieved 10 or 20 seconds per year in Grand Quartz and King Quartz watches.

Daini Seikosha used a different approach, averaging the timing between two crystals with different temperature characteristics. This also delivered excellent accuracy, especially in later movements like their Cal. 9682 which was also good to 5 seconds per year.

• Seiko 99 (1978-1985) - Suwa
  • Seiko 9920/9921/9923 (1978-1985) - 20 seconds per year, Seiko King Quartz
  • Seiko 9940/9942/9943 (1978-1985) - 10 seconds per year, Seiko Grand Quartz
  • Seiko 9980/9983 (1978-1985) - 5 seconds per year, Seiko Superior
• Seiko 92 (1978-1985) - Daini
  • Seiko 9223 (1981-1985) - 10 seconds per year, Seiko King Quartz
  • Seiko 9256 (1978-1980) - 10 seconds per year, Seiko Grand Quartz
• Seiko 97 (1979-1981) - Daini
  • Seiko 9721/9722/9723/9726 (1979-1981) - 20 seconds per year, Seiko King Quartz
• Seiko 96 (1979-1981) - Daini
  • Seiko 9641/9642 (1980-1983) - 20 seconds per year, Seiko King Quartz, Seiko Dolce
  • Seiko 9661 (1980-1983) - 20 seconds per year, Credor
  • Seiko 9681/9682 (1981-1985) - 5 seconds per year, Seiko Superior
• Seiko 94 (1980-19??) - Suwa
  • Seiko 9441/9442/9443 (1980-1985) - 20 seconds per year, Seiko King Quartz, Seiko Dolce, Seiko Lassale
  • Seiko 9461 (1981) - 10 seconds per year, Seiko Grand Quartz
  • Seiko 9481/9483 (1980-1985) - 5 seconds per year, Seiko Superior