Reference for Shabbat 17:4
וכי תימא דלית ביה ד' על ד' והאמר רב חמא בר גוריא אמר רב תוך הפתח אע"פ שאין בו ד' על ד' צריך לחי אחר להתירו
'Others state, A threshold serves as two domains: if the door is open, it is as within; if the door is shut, it is as without.'<span class="x" onmousemove="('comment',' Rashi: this is now assumed to refer to a threshold lying at the opening of a blind alley between it and the public road. An alley was made fit for carrying by planting a stake at the side of the opening, which by a legal fiction was regarded as a complete partition stretching right across, and it is understood that this threshold is excluded from the partitioning influence of a stake, which was fixed at the inner side of the threshold. Tosaf. explains it somewhat differently. ');"><sup>4</sup></span>