Bridge Chestnut 4 - "Eight ever, nine never"
This chestnut comes from whether you should play Ace and King when you are missing the Queen or whether you should finesse.
With 8 trumps here, the odds favour finessing for the Queen rather than playing Ace and King and hoping that the Queen drops doubleton.
With 9 trumps, the odds favour playing the Ace and King, expecting the Queen to drop singleton or doubleton. Be warned, playing for the "drop" is only marginally better than finessing
In each case, you should give yourself an extra chance. Advance the Jack first. A sleepy RHO may play the Queen and solve your problem.
The main problem with this chestnut is that it presumes that you have no other knowledge.
This is rarely the case :-
However, the most common reason to play Ace and King even though you have only eight trumps is that you cannot afford to lose the lead while there are trumps out. The opening lead and play has convinced you that an opponent will get a ruff.
Be aware that, when you breach this chestnut, you may be heading for a different score to the rest of the field. Most will follow this chestnut. You need to be prepared to back your judgment!