Trikonasana (Triangle Pose), so named for the shape the body takes when in the asana, builds strength and lengthens the thighs, knees, and ankles.
Trikonasana opens the hips, groins, hamstrings, shoulders, chest, and lengthens the spine.
It builds core strength and balance.
Build this asana into a warrior sequence - moving from Virabhadrasana II (Warrior Pose II), into Trikonasana: remaining in a wide-leg stance (with front foot heel and back foot arch aligned), straighten both legs, extend the torso over one leg and ease the body down, reaching up with one arm and hand and reaching down to shin, ankle or the floor with the other. Draw the bottom in alignment with the plane of the body and legs. (Many years ago I was told to imagine holding the pose between two panes of glass.)
Avoid dumping weight into the lower arm and shoulder, keeping the weight of the upper body spread across the arm reaching up and away from the body and the other down to the floor, while opening up through the chest.
This is a strong pose for tight hamstrings (and you may find one hamstring may have more length than the other); soften through the knees until you have more flexibility and ease of movement.
With an inhale, draw the body back up to standing, or take both hands down to the mat, twist the back foot around so the toes are pointing to the front of the mat and move into Adho Mukha Svanasana (down dog) or into Phalakasana (High plank pose).