# Concatenation files

Concatenation "conc" files are simple txt files that offer a simple way of specifying a list of imaging files that should be concatenated when being processed or analyzed. They can represent a sequence of BOLD runs that are to be concatenated when extracting data or computing analyses, and they can list the individual results files that are to be combined when performing second level statistics. In all cases conc files enable virtual concatenation of image files without creating a new data file. Each conc file just lists references to the files that are to be combined in the required order.

Conc files are most often generated for the specific analysis goal. For running activation analyses and functional connectivity processes they are placed into `<study folder>/sessions/inbox/concs` folder. Conc files that list the resulting files are found in `<study folder>/sessions/<session id>/images/functional/concs`.

Conc files have the following structure: The first line specifies the number of files listed, and each other line lists a path to a single imaging file to be concatenated. The number of files is specified using keyword `number_of_files:` each file is then preceded by keyword `file:`.

## Example

``` bash
number_of_files: 3
    file:/data/study1/subject3/bold/bold1.nii.gz
    file:/data/study1/subject3/bold/bold4.nii.gz
    file:/data/study1/subject3/bold/bold7.nii.gz
```

When conc files are provided as input to QuNex MATLAB scripts, all the files listed will be loaded into a single image object in the order in which they are listed in the conc file.

By convention conc files end with `.conc` extension.

To ease generation of `.conc` files, a [`create_conc`](../../api/gmri/create_conc.rst) QuNex command is available. The command can be called with a set of parameters that allow flexible generation of `.conc` files for a specific set of sessions and the desired set of files. For details, please consult in-line documentation by running `qunex ?create_conc`.