I hadn't been able to trace my maternal grandfather back at all. Didn't know his birthdate, didn't know where he'd been born, although apparently my maternal grandmother would disparagingly refer to him as a Geordie when she was mad at him. No marriage certificate, since grandfather and grandmother had met in Birmingham and run off to Manchester when grandmother got pregnant. All I had was a death certificate, which might or might not have had his correct age. My mother was born in 1930, so no census data was available.
My mother had told me years ago that when she was just a lass, a woman had come to their house and wanted to talk to her father. The woman, accompanied by a young woman who was announced as 'this is your sister', called herself Mrs. X.
Finally the 1939 Register came out, and I was able to get my grandfather's birth date. But I still couldn't find anything that seemed to match up with him for a birth certificate. So I started thinking about the woman who had called herself Mrs. X. Was 'X' his real name, instead of 'Y'?
And following that, everything fell into place. He'd been born in Birmingham, but his parents were from Newcastle on Tyne. He couldn't marry my grandmother, because he was already married (and had several children!).
The kicker and tie-in? The name he used as his alias, 'Y'? I wondered how he had decided on that. Turned out to be his mother's maiden name!
Not the most sterling character, but at least I know who he was, all due to a half-remembered memory of my then 9 year old mother.