ENH: Clarify the relation of motion.tsv columns to channels.tsv rows (b…

…ids-standard#1699)

* ENH: Clarify the relation of motion.tsv columns to channels.tsv rows

* ENH: Add motion as exception to TSV rules in common principles

src/common-principles.md

@@ -430,7 +430,8 @@ Tabular data MUST be saved as tab delimited values (`.tsv`) files, that is, 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 of
-[physiological and other continuous recordings](modality-specific-files/physiological-and-other-continuous-recordings.md)).
+[physiological and other continuous recordings](modality-specific-files/physiological-and-other-continuous-recordings.md)
+as well as [motion recording data](modality-specific-files/motion.md)).
 It is RECOMMENDED that the column names in the header of the TSV file are
 written in [`snake_case`](https://en.wikipedia.org/wiki/Snake_case) with the
 first letter in lower case (for example, `variable_name`, not `Variable_name`).

src/modality-specific-files/motion.md

@@ -42,7 +42,10 @@ each of which is accompanied by `*_tracksys-<label>_motion.json` and `*_tracksys
 Between `tracksys-<label>` entity and `*_motion.tsv`, `*_motion.json`, or `*_channels.tsv` suffixes, optional [`acq-<label>`](../appendices/entities.md#acq) or [`run-<index>`](../appendices/entities.md#run) entity MAY be inserted.
 
 One column in the `*_tracksys-<label>_motion.tsv` file represents one data channel.
-The ordering of columns MUST match the order of rows in the `*_channels.tsv` file for unambiguous assignment.
+Motion files MUST NOT have a header row;
+the ordering of columns is given by the order of rows in the associated `*_channels.tsv` file.
+The number of columns in `_motion.tsv` files MUST equal the number of rows
+in the associated `_channels.tsv` file.
 All relevant metadata about a tracking systems is stored in accompanying sidecar `*_tracksys-<label>_motion.json` file.
 
 The source data from each tracking system in their original format, if different from `.tsv`,