Why is the socket number incremented by one in libssh2 x11 forwarding example?

I am looking to x11 forwarding example in libssh2 source (example/x11.c). In the function x11_send_receive I see this code:

rc = select(sock + 1, &set, NULL, NULL, &timeval_out);
if(rc > 0) {
    memset((void *)buf, 0, bufsize);

    /* Data in sock*/
    rc = read(sock, buf, bufsize);
    if(rc > 0) {
        libssh2_channel_write(channel, buf, rc);
    }
    else {
        free(buf);
        return -1;
    }
}

I don't understand why the socked is incremented (+ 1) in the select statement. Can someone explain this?

Probably it is related to listening sockets - I see two:

# netstat -lxn | grep /tmp/.X11-unix/X0
unix  2      [ ACC ]     STREAM     LISTENING     32746    /tmp/.X11-unix/X0
unix  2      [ ACC ]     STREAM     LISTENING     32745    @/tmp/.X11-unix/X0

sock is connecting to path /tmp/.X11-unix/X0 (without @)... So what is in the background?...