There is also https://catalog.ldc.upenn.edu/docs/LDC2004T12/RTTM-format-v13.pdf which differs from the spec given by @bernlim mostly in the omission of the 10-th field “SLAT”. I found that some tools indeed do not output 10 but 9 fields, and that md-eval-v21.pl used for scoring diarizations does not bother to read the 10-th field.

RTTM

Rich Transcription Time Marked (RTTM) files are space-delimited text files containing one turn per line, each line containing ten fields:

• Type — segment type; should always by SPEAKER
• File ID — file name; basename of the recording minus extension (e.g., rec1_a)
• Channel ID — channel (1-indexed) that turn is on; should always be 1
• Turn Onset — onset of turn in seconds from beginning of recording
• Turn Duration — duration of turn in seconds
• Orthography Field — should always by < NA >
• Speaker Type — should always be < NA >
• Speaker Name — name of speaker of turn; should be unique within scope of each file
• Confidence Score — system confidence (probability) that information is correct; should always be < NA >
• Signal Lookahead Time — should always be < NA >

For instance:

SPEAKER CMU_20020319-1400_d01_NONE 1 130.430000 2.350 <NA> <NA> juliet <NA> <NA>
SPEAKER CMU_20020319-1400_d01_NONE 1 157.610000 3.060 <NA> <NA> tbc <NA> <NA>
SPEAKER CMU_20020319-1400_d01_NONE 1 130.490000 0.450 <NA> <NA> chek <NA> <NA>

To write rttm file:

with open(rttmf, 'wb') as f:
    for turn in turns:
        fields = ['SPEAKER', turn.file_id, '1', format_float(turn.onset, n_digits), format_float(turn.dur, n_digits),
              '<NA>', '<NA>', turn.speaker_id, '<NA>', '<NA>']
        line = ' '.join(fields)
        f.write(line.encode('utf-8'))
        f.write(b'n')

reference urls:
https://github.com/nryant/dscore
https://github.com/nryant/dscore/blob/824f126ae9e78cf889e582eec07941ffe3a7d134/scorelib/rttm.py#L103