Beginner Left Thumb Position

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Ive just started bass and been doing a few lessons with a teacher. anyways my teacher has me doing these excercises to get strength into my fingers and make my reach better. it involves me just putting my 1st finger on the 1st fret 2nd finger on 2nd friend and so on. he also tells me to keep my thumb positioned behind the second fret but my thumb sorta twists if thats the right word to describe it when i play the 4th finger on the 4th fret. i will go into greater detail if that is too hard to understand but iwas just wondering if this minor twist of the thumb is wrong or not.

[Howlin' Hanson]

Just my opinion, but your fingering will work best if your thumb is directly opposed to your middle finger (no matter what fret the middle finger is playing on). Look at the thumb as your pivot point.

Having said that, I have seen video of even the great Entwistle flopping his thumb on top of the fretboard. So no rule is inviolable.