In a key-sequenced data set, logical records are placed in the data set in ascending collating sequence by a field, called the key. Figure 1 shows that the key contains a unique value, such as an employee number or invoice number, that determines the record's collating position in the data set.

Figure 1. Record of a Key-Sequenced Data Set

The key must be in the same position in each record, the key data must be contiguous, and each record's key must be unique. After it is specified, the value of the key cannot be altered, but the entire record can be erased or deleted. For compressed data sets, the key itself and any data before the key will not be compressed.

When a new record is added to the data set, it is inserted in its collating sequence by key, as shown in Figure 2.

Figure 2. Inserting records into a key-sequenced data set