A single thread of control - the main thread of the process. In multi-threaded process, you will need a list to keep the threads belonging to that process. Other objects, such as open file descriptors.
Nachos provides threads.
The Nachos scheduler maintains a data structure called ready list, which keeps track of the threads that are ready to execute. The thread is currently running. A blocked thread does not reside on the ready list. This state is a temporary state used during thread creation. Thread's context block - information associated with thread, is maintained as private data of a Thread object instance.