Once oxen started to be replaced by horses on farms and in fields, the yoke became obsolete due to its shape not working well with a horses' posture. The first design for a horse collar was a throat-and-girth-harness. These types of harnesses were unreliable though due to them not being sufficiently set in place. The loose straps were prone to slipping and shifting positions as the horse was working and often caused asphyxiation. Around the eighth century, the introduction of the rigid collar eliminated the problem of choking. The rigid collar was placed over the horses head and rested on its shoulders. This permitted unobstructed breathing and placed the weight of the plow or wagon where the horse could best support it.