for joining tubing to something else or joining two lengths of tubing (apparentlyh a variant of variant of knelfitting in the plumbing context)

For knefitting and compression fitting, see the numerous hits with google image searching.

Knelsluiting doesn't yield any images, but it does yield a few text hits. The following shows example of compression fittings:

http://www.aquariaveldhuis.nl/query2.php?B=163&C=79

There is also one hit with a telescopic tubing using compression fittings to secure the tubing at the desired extension, and an 'outsider' that uses a different sort of compression fitting for securing a guard railing (but it doesn't have to provide a gas-tight or water-tight seal).