A two-arms clinical trial is a type of clinical study where participants are divided into two groups (or “arms”):

1. Treatment Group (Arm 1): Participants receive the experimental treatment or intervention.
2. Control Group (Arm 2): Participants may receive a placebo, standard treatment, or no intervention.

These trials are common in evaluating the efficacy and safety of new drugs, medical devices, or treatment protocols by directly comparing outcomes between the two arms.

Multi-arm designs are study frameworks used primarily in clinical and biomedical research where more than two groups (or "arms") are compared simultaneously.