Herm Sprenger Ultra-Plus Prong Training Collar, 3.80 mm. Total Length: 20" Herm Sprenger Ultra-Plus Prong Dog Training Collar, chain - 3.8 mm. Total length: 20". Adjustable sizing to fit necks 17" to 20.0". This training collar features a solid fastener plate that provides a more secure connection making fastening the collar around your dog's neck easier and safer. In cases when additional size adjustment is required individual links can be added or removed. The patented center plate construction and symmetrical prong placement provide natural contact for optimal communication. The collar is safe, practical and easy to use.