Best Flexibility Yoga?

I agree with those who say Hatha and Yin. Essentially you need long holding of asanas to develop flexibility.

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It depends what part of the body your trying emphasize. Flexibility in Yoga can be very simple and basic as just stretching your legs. Are you working on your back, legs, arms?

Camel pose is a good flexibility practice. I used this pose to help arch my back when I was working on my Scorpion pose. Every day you can work on bringing your back/head down, down and down..