RRSTDV8 wrote:

The body, engine etc. are all "grounded" to the negative terminal. If you undo the positive first and, for example, allow your spanner to touch metal whilst undoing the positive terminal you create a circuit. By undoing the negative first, if your spanner touches anything on the body etc., it doesn't matter. When you then undo the positive, it doesn't matter because the negative is no longer connected to the body/engine so no circuit occurs.

Likewise when reconnecting, the positive is connected first for the same reason.

That's my understanding of it, anyway.

Oh, and it's worth remembering that some older cars (e.g. some classics from back in the 50s and 60s) are positive earth - the bodywork is grounded to the positive terminal. On these, you do the process in reverse by taking off the positive first.