That was my understanding too, but I wanted to check. No wonder the joint leaked. I checked for an olive that had welded itself to the bevel in the compression fitting and had pulled off the pipe, but there was none. It's a miracle that the fitting had been watertight for the last
47 years that my parents have lived in the house, though I suppose some pipework may have been renewed since then.I found a large bag of pipe fittings in my dad's shed when I was sorting it through (he died earlier this year) but although there were plenty of solder fittings, the only compression one was a 22 mm straight through with 15 mm branch, rather than 15 mm straight through and branch. I could have gone out and bought a new one (with olive!) but I thought, wrongly, that my temporary tightening would at least last long enough for mum to call a plumber to do a permanent job.