Why a sequence has 4 lines in fastq format

[gene_x]

As you know, the general format for fastq files have four lines:
1st: @ with optional sequence identifier
2nd: sequence
3rd: + with optional sequence identifier
4th: quality score for sequence in 2nd line

The 3rd line doesn't really convey any useful information... do you know why is it included?

[dkatzel]

the format doesn't actually require 4 lines. Valid fastq files can be more than 4 lines with the sequence and quality values spread of several lines each. The quality header is used to tell you "there's no more sequence data left". Also since there are several quality encodings, it would be hard to tell where the quality starts otherwise if you didn't know the quality encoding.

[JohnN]

As you know, the general format for fastq files have four lines:
1st: @ with optional sequence identifier
2nd: sequence
3rd: + with optional sequence identifier
4th: quality score for sequence in 2nd line

The 3rd line doesn't really convey any useful information... do you know why is it included?

To make it easy for lazy programmers like me to know when bases have stopped and quality scores have started? Especially when line feeds can't be trusted?

[maubp]

FASTQ was invented a long time ago, and was initially used with longer Sanger reads where line wrapping was a nice feature.

See also http://dx.doi.org/10.1093/nar/gkp1137

[gene_x]

FASTQ was invented a long time ago, and was initially used with longer Sanger reads where line wrapping was a nice feature.

See also http://dx.doi.org/10.1093/nar/gkp1137

What is this line wrap? Is it something like \n ?