Even though the title of this paper suggests a tutorial on large mainframe computers, this is not quite the case. Obviously, anyone who has, or contemplates having, a large mainframe computer does not need a tutorial on the subject. However, the common thread through this paper will be the large mainframe computer, which is the heart of a commercial timeshared system or a commercial computer packet switching network. Thus, we will discuss some aspects of the use of large mainframe computers either in-house or at a distance through a commercial timeshared network or a packet switching network. The common access mode which is the computer terminal will also be discussed.