My mum is 'Nan'. My dad died of cancer when he was very young (24) and mum remarried a few years later when I was seven. I have always called my step-father John, and he gets called 'Nandad' by my kids

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The kids' mum's parents were divorced when we met and her dad had remarried. Ros' dad (who has since passed away) was 'Grandpa' and his wife 'Anne-ma' for a while, but when they got older they used to just call her 'Anne'.
Ros' mum is 'Grandma' and her husband Len is 'Grandad'.
I used to call my mum's parents Granma and Granda, but you really have to read it with a Geordie accent to get an idea of what it sounded like.
My dad's dad also died very young, and I don't remember him at all. My dad's mum was 'Granma Doris'.
Confused? I am...
Mick