A connection for percussion drilling, comprising: a male coupling (1, 9), comprising: a body; and a male screw thread (1t, 9t) formed on an outer surface of the body, and a female coupling (2, 6) comprising: a body; and a female screw thread (2t, 6t) formed in an inner surface of the body, wherein: each thread has a thread-form (4m, 4f, 10m, 10f) including a crest (A 4), a root (A 1), a contact flank (E 1) and a non-contact flank (E 2), each thread-form (4m, 4f, 10m, 10f) has a contact flank angle (α) and a non-contact flank angle (β) inclined relative to a respective baseline 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 (4m, 4f, 10m, 10f) 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 (4m, 4f, 10m, 10f) 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 (1t, 9t) formed on an outer surface of the body, and a female coupling (2, 6) comprising: a body; and a female screw thread (2t, 6t) formed in an inner surface of the body, wherein: each thread has a thread-form (4m, 4f, 10m, 10f) including a crest (A 4), a root (A 1), a contact flank (E 1) and a non-contact flank (E 2), each thread-form (4m, 4f, 10m, 10f) has a contact flank angle (α) and a non-contact flank angle (β) inclined relative to a respective baseline 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 (4m, 4f, 10m, 10f) 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 (4m, 4f, 10m, 10f) 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 (4m, 4f, 10m, 10f) including a crest (A 4), a root (A 1), a contact flank (E 1) and a non-contact flank (E 2), each thread-form (4m, 4f, 10m, 10f) has a contact flank angle (α) and a non-contact flank angle (β) inclined relative to a respective baseline 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 (4m, 4f, 10m, 10f) 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 (4m, 4f, 10m, 10f) 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).