What if you arse around on your instrument to find the note you think you want and end up playing Mary Had A Little Lamb in the key of C (to keep it simple)?

You go down to your local session and hey, that's the tune they start playing. But (being a session) they're doing it in D. But you don't know that so what you've worked out sounds horrendous (actually it doesn't but that's not quite the point). But you've still no idea why the Fs and Cs have been sharpened because you've started on the 'wrong' note. So you just do it to compensate but have no idea why because you don't know what intervals are.

A small amount of basic music theory is scarcely rocket science and saves an immense amount of time and grief. Ruth's daughter could keep a filo with manuscript paper on which to write down the first few bars of tunes with key and time signatures. And a surer way of knowing you need to transpose (for beginners and me anyway), is not so much by ear but by watching where your neighbours are putting their fingers.

Ear players generally frown at a little book and look blankly when asked what key they're proposing to play in. But if it saves your corner from sounding like a subsection of the Portsmouth Sinfonia (why are they invading every thread?), then it is surely so much the better.