Hi Declan..The "centre 3/4 rule applies to tread..tread is defined by having a tread wear indicator in the "channel"..if it doesn't have then its called a "sipe"..For instance some tyres only have twi's in the small "channel" in the centre of the tyre..the rest of the tyre can be completely bald and still pass an mot..frustrating i know even for us testers..Gary