What do _._1 and _++_ mean in Scala? _._1 _1 is a method name. Specifically tuples have a method named _1, which returns the first element of the tuple. So _._1 < _._1 means “call the _1 method on both arguments and check whether the first is less than the second”.
Example .toList.sortWith(_._1 < _._1)
_._1 calls the method _1 on the wildcard parameter _, which gets the first element of a tuple.