The below is taken directly from the BIDS specification (version 1.1.1); please see this for full details. Screaming in the original.

4.2 Tabular files

Tabular data MUST be saved as tab delimited values (.tsv) files, aka csv files where commas are replaced by tabs. Tabs MUST be true tab characters and MUST NOT be a series of space characters. Each TSV file MUST start with a header line listing the names of all columns (with the exception for physiological and other continuous acquisition data - see below for details). Names MUST be separated with tabs. String values containing tabs MUST be escaped using double quotes. Missing and non applicable values MUST be coded as “n/a”.

4.2.1 Example:1

onset duration response_time correct stop_trial go_trial

200 20 0 n/a n/a n/a

Tabular files MAY be optionally accompanied by a simple data dictionary in a JSON format (see below). The data dictionaries MUST have the same name as their corresponding tabular files but with .json extensions. Each entry in the data dictionary has a name corresponding to a column name and the following fields:

• LongName - Long (unabbreviated) name of the column.
• Description - Description of the the column.
• Levels - For categorical variables: a dictionary of possible values (keys) and their descriptions (values).
• Units - Measurement units. [] format following the SI standard is
• RECOMMENDED (see Appendix V).

• TermURL - URL pointing to a formal definition of this type of data in an ontology available on the web.

Further examples are provided in the specification.