Source Audrey Grant Better Bridge
When you are the first or second player to make a call in the auction, the strength of partner’s hand is unlimited, since partner has not yet had a chance to speak. If you open, partner will expect a hand worth 13 or more points and bid accordingly. Holding 13 or more points, responder will push the partnership to at least the game level.
The following guideline can be used when you have a close decision:
1) 
A J 7 5 3 2) A J 7 5
K 4 K 4 3
Q 10 7 3 Q 10 7 3
J 5 J 5
Following this guideline, you would open the first hand (11 + 5 + 4 = 20) but not the second (11 + 4 + 4 = 19).
Rule of 20: In borderline cases in first or second position, add your high-card points to the number of cards in your two longest suits. If the total is 20 or more, consider opening.