Reference: Dryer and Haspelmath (eds.) 2013

Dryer, Matthew S. & Martin Haspelmath (eds.) 2013. WALS Online. Leipzig: Max Planck Institute for Evolutionary Anthropology.

https://wals.info/

@book{wals,
  address   = {Leipzig},
  editor    = {Matthew S. Dryer and Martin Haspelmath},
  publisher = {Max Planck Institute for Evolutionary Anthropology},
  title     = {WALS Online},
  url       = {https://wals.info/},
  year      = {2013}
}