# Folders¶

The BIDS format is essentially a way to structure your data / metadata within a hierarchy of folders. This makes it easy to browse from a computer, as well as to automatically parse a BIDS folder with a program. The BIDS structure makes minimal assumptions about the tools needed to interact with the data that’s inside.

These are the three main types of files you’ll find in a BIDS dataset:

1. .json files that contain key: value metadata

2. .tsv files that contain tables of metadata

3. Raw data files (for example: .jpg files for images or .nii.gz files for fMRI data.)

These three types of files are organized into a hierarchy of folders that have specific naming conventions. The rest of this page describes how these folders are structured.

## Overview¶

There are four main levels of the folder hierarchy, these are:

project/
└── subject
└── session
└── datatype


With the exception of the top-level project folder, all sub-folders have a specific structure to their name (described below). Here’s an example of how this hierarchy looks:

myProject/
└── sub-01
└── ses-01
└── anat


Here is the folder name structure of each level:

## project¶

Can have any name, this should be descriptive for the dataset contained in the folder.

## subject¶

Structure: sub-<participant label>

One folder per subject in this dataset. Labels should be unique for each subject.

## session¶

Structure: ses-<session label>

In general, a session represents a recording session, and subjects will stay in the scanner or headset during that session. You might have multiple sessions per subject if you collected data from them on several occasions. If there is only a single session per subject, this level of the hierarchy may be omitted.

For more details, refer to this section of the FAQ.

## datatype¶

Represents different types of data. Must be one of:

• func

• dwi

• fmap

• anat

• meg

• eeg

• ieeg

• beh

• pet

• micr

The name for the datatype depends on the recording modality.

modality
datatype mri pet meeg behavioral microscopy
anat pet eeg beh micr
func
meg
dwi ieeg
perf

## BIDS folder example¶

Below is the folder hierarchy for one of the BIDS example datasets. It has multiple subjects of data, and includes metadata files (.tsv and .json) both between- and within-subjects.

Note that it has one session per subject, so this level is omitted.

ds001
├── dataset_description.json
├── participants.tsv
├── sub-01
│   ├── anat
│   │   ├── sub-01_inplaneT2.nii.gz
│   │   └── sub-01_T1w.nii.gz
│   └── func
├── sub-02
│   ├── anat
│   │   ├── sub-02_inplaneT2.nii.gz
│   │   └── sub-02_T1w.nii.gz
│   └── func
...
...


## Creating a BIDS folder hierarchy¶

Next we’ll step through a sample process one might follow when creating a BIDS hierarchy for a new dataset.

### Create the folder hierarchy and top-level metadata file¶

First we’ll create the folder hierarchy to be used in this format.

• Create the top-level project folder (myProject/)

• Create a top-level metadata files (myProject/participants.tsv and myProject/task-mytask.json)

### Create a subject’s folder¶

Next we’ll add the folder hierarchy for one subject:

• Create the <Project>/<Subject>/<Session>/ folder (myProject/sub-01/ses-01/)