Reference for Eruvin 178:11
כהא דאמר רב נחמן עשה סולם קבוע לגגו הותר בכל הגגין כולן
only were there was a partition<span class="x" onmousemove="('comment',' All round the roofs except where they adjoin one another.');"><sup>26</sup></span> on the one as well as on the other, since the use of the bigger roof is rendered permissible by the railings,<span class="x" onmousemove="('comment',' Or 'side-posts', sc. the imaginary upward extensions of its projections on either side of the smaller roof (cf. Supra p. 622, n. 19) .');"><sup>27</sup></span> while [the use of the smaller one is forbidden since] it has a breach extending along its entire length, but if there was no partition either on the one or on the other, the use of both is forbidden'.<span class="x" onmousemove="('comment',' The imaginary railings or side-posts being of no avail where no partitions exist with which to form a doorway.');"><sup>28</sup></span>