Any brand of yarn that knits to the same tension should be fine.
Look at the pattern, a rough guide is that double knit will usually use a 4mm needle, 4 ply a 3.25mm needle. The bigger the needle the thicker the yarn.
Your gran will need to knit a tension square, and adjust the size of needle to suit - the pattern will explain how to do this.
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Some days I pray for silence
Some days I pray for soul
Some days I just pray to the god of sex and drums and rock 'n' roll
