A connection for percussion drilling, comprising: a male coupling , comprising: a body; and a male screw thread formed on an outer surface of the body, and a female coupling comprising: a body; and a female screw thread formed in an inner surface of the body, wherein: each thread has a thread-form including a crest (A 4 ), a root (A 1 ), a contact flank (E 1 ) and a non-contact flank (E 2 ), each thread-form has a contact flank angle (α) and a non-contact flank angle (β) inclined relative to a respective baseline (BL) located at a respective minor diameter (D N ) or major diameter (D J ) thereof, each non-contact flank angle (β) is greater than the respective contact flank angle (α), the crest (A 4 ) of each thread-form is inclined from the respective contact flank (E 1 ) to the respective non-contact flank (E 2 ) such that an apex (X A ) of the respective thread-form defining the respective major diameter (D J ) and minor diameter (D N ) thereof is located adjacent to the respective non-contact flank (E 2 ) characterized in that : each root (A 1 ) is a first arc, and each contact flank (E 1 ) is connected to the respective root (A 1 ) by a respective second arc (A 2 ) wherein a first radius (R 1 ) of each first arc is greater than a second radius (R 2 ) of the respective second arc (A 2 ). a body; and a male screw thread formed on an outer surface of the body, and a female coupling comprising: a body; and a female screw thread formed in an inner surface of the body, wherein: each thread has a thread-form including a crest (A 4 ), a root (A 1 ), a contact flank (E 1 ) and a non-contact flank (E 2 ), each thread-form has a contact flank angle (α) and a non-contact flank angle (β) inclined relative to a respective baseline (BL) located at a respective minor diameter (D N ) or major diameter (D J ) thereof, each non-contact flank angle (β) is greater than the respective contact flank angle (α), the crest (A 4 ) of each thread-form is inclined from the respective contact flank (E 1 ) to the respective non-contact flank (E 2 ) such that an apex (X A ) of the respective thread-form defining the respective major diameter (D J ) and minor diameter (D N ) thereof is located adjacent to the respective non-contact flank (E 2 ) characterized in that : each root (A 1 ) is a first arc, and each contact flank (E 1 ) is connected to the respective root (A 1 ) by a respective second arc (A 2 ) wherein a first radius (R 1 ) of each first arc is greater than a second radius (R 2 ) of the respective second arc (A 2 ). each thread has a thread-form including a crest (A 4 ), a root (A 1 ), a contact flank (E 1 ) and a non-contact flank (E 2 ), each thread-form has a contact flank angle (α) and a non-contact flank angle (β) inclined relative to a respective baseline (BL) located at a respective minor diameter (D N ) or major diameter (D J ) thereof, each non-contact flank angle (β) is greater than the respective contact flank angle (α), the crest (A 4 ) of each thread-form is inclined from the respective contact flank (E 1 ) to the respective non-contact flank (E 2 ) such that an apex (X A ) of the respective thread-form defining the respective major diameter (D J ) and minor diameter (D N ) thereof is located adjacent to the respective non-contact flank (E 2 ) characterized in that : each root (A 1 ) is a first arc, and each contact flank (E 1 ) is connected to the respective root (A 1 ) by a respective second arc (A 2 ) wherein a first radius (R 1 ) of each first arc is greater than a second radius (R 2 ) of the respective second arc (A 2 ). each root (A 1 ) is a first arc, and each contact flank (E 1 ) is connected to the respective root (A 1 ) by a respective second arc (A 2 ) wherein a first radius (R 1 ) of each first arc is greater than a second radius (R 2 ) of the respective second arc (A 2 ). <EOS>