The hidden, unspoken, unwritten, knowledge that we all possess, which we cannot pass on through writing, like riding a bike, or recognizing a face. This is knowledge that can only be transferred by face-to-face contact (Sternberg in R. Kitchin and N. Thrift, eds 2009). Howells (2002) Urb. Studs 39, 5–6, 872 is very helpful on this.