Thread Sewn
It is a similar binding to perfect bound. However, perfect bound is not suitable for long term usage because the glue hardens by alternating cold and hot weather. Instead of perfect bound, thread sewn employs multiple threads to sewn the signatures together. Since the threads are flexible and aren’t affected by weather, the books are much more durable.
It may look similar to perfect bound but it has a few differences. The book can also be open wide on the table. The threads appear at the middle of the signature.