Q. Explain about Double Error Detection bit?

Let's presume now that two bit errors take place in data.

Data received:

So on -matching we conclude P3-D3 pair doesn't match.

Despite the fact that this information is wrong. Such problems can be recognized by adding one more bit to this Single Error Detection Code.  This is known as Double Error Detection bit (P5, D5).

So our data now is

Data receiving end.

D5-P5 mismatch denotes there is double bit error therefore don't try to correct error in its place asks sender to transmit data again.  So the name single error correction but double error detection as this code corrects single bit errors however only detects error in two bit.