The Suit Quality Test guide measures how strong a suit you hold. The Suit Quality or SQ often decides whether the suit is strong enough to bid at a given level. Useful just about whenever suit quality is a relevant criterion, it is useful for pre-empts, weak twos and overcalls. The SQ determines whether you can afford to insist on your suit as trumps.
Count the number of honors in the long suit and then add the total number of cards in the long suit. The answer is the SQ of that suit. For example, Q-8-7-4-2 has 1 honor and 5 cards. 1 + 5 = SQ of 6.
The SQ should equal or exceed the number of tricks for which you are bidding for an overcall. With an SQ of 6, such a suit is not good enough for an overcall even at the one-level (seven tricks requires an SQ of 7). K-J-9-5-4 has an SQ of 7 (2 honors + 5 cards). That is good enough to overcall at the one-level (assuming you have enough extra strength) but not strong enough for a two-level overcall.
Count the J or 10 as an honor card for the SQ Test only if you also hold a higher honor in the long suit.
J-8-5-4-3-2 has an SQ of 6. Do not count the jack if you have no higher honor.
K-J-8-4-3-2, however, has an SQ of 8. The jack is full value as an honor because the king is also present.
The Rule of 10 is a useful product of the Suit Quality Test. You may insist on that suit as trumps if you have a suit with an SQ of 10,. For example, suppose you hold:
K Q J 10 6 4
Q 3 2
Partner opens 1D , you bid 1H and partner rebids 2D . What now?
The heart suit has an SQ of 10 (4 honors + 6 cards). Therefore, you should insist on hearts as trumps and jump to 4H . Even if partner is void in hearts, chances are you will lose only one trump trick.
Change the hand to:
K Q 8 6 4 3
Q J 10
Now, after 1D : 1H , 2D , you should bid only 3H . That is a strong rebid and shows six hearts, but the suit needs support from partner. You would not fancy being in 4H opposite a void or low singleton.