The Rule of Fifteen says that in Fourth Seat after three passes if you are in a borderline situation considering whether to open light, say 10 to 12 points, then you should add your high card points to the number of Spades you hold and if less than 15 you should pass the hand out.
The logic behind this states that when there are three passes to you in Fourth Seat the high card points are likely to be evenly divided between the two pairs
The probability is then that the opposition have Spades and they will win the Auction
Some say you should only open if you have a Spade suit.This seems very restrictive but as a general rule the advice given by many experts is simply whether to open any suit.
Two examples where it worked (As in all things Bridge it will get you a bad board occasionally)
♠Qxx
♥KJx
♦xxxxx
♣Qx
♠Kxx ♠AJxxx
♥xxx ♥Qxx
♦KJxx ♦Ax
♣Kxx ♣xxx
♠xx
♥Axxx
♦Qx
♣AJxxx
West to open and the bidding goes round to South. 11 points and 2 Spades ,makes 13 so South should pass. If they open a Club North will bid a Diamond , East will now bid a Spade and EW will play in 1 or 2 Spades, making comfortably.
♠xx
♥Qxx
♦Jxx
♣AKxxx
♠Axx ♠KJxx
♥KJx ♥xxxxx
♦xxx ♦Ax
♣Jxxx ♣Qx
♠Qxxx
♥Ax
♦KQxxx
♣xx
Now South has 11 points and 4 Spades making 15 so they can open a Diamond, North will bid 2 Clubs, East hasnt got a sensible bid, and South will bid 2 Diamonds which will probably be passed out.
You can of course construct many hands where the opposite will apply but the experts advice is that on balance you are better off following this rule.