This is a text description of onliner2.gif.  

This figure shows two disks, DISK A and Disk B, containing online redo log 
files. Each disk contains identical online redo log files, that are written to 
concurrently (multiplexed) by LGWR. Each identical set of redo log files is 
called a group. This figure shows two groups. 

Group 1 consists of:
1) A_LOG1 on DISK A
2) B_LOG1 on DISK B

Group 2 consists of:
1) A_LOG2 on DISK A
2) B_LOG2 on DISK B