P10

From Simplex

PRINCIPLE 10: Responding hands with trump support but less than 10 HCP bid immediately to their total trump level. (For example, after a 1♥ opening by partner, a hand containing 6 HCP and four cards in the heart suit would immediately raise to 3♥. Partner is known to have at least five hearts. Add those five to the four that responder has makes nine. Take six from nine means a raise to the 3-level.) Equally if opener has less than 10 HCP but support for responder's suit, he will bid to the total trump level on the second round.