Talmud for Shabbat 223:9

מאי היא דתניא סנדל שנפסקו שתי אזניו או שתי תרסיותיו או שניטל כל הכף שלו טהור אחת מאזניו או אחת מתרסיותיו או שניטל רוב הכף שלו טמא ר' יהודה אומר נפסקה פנימית טמא החיצונה טהור ואמר עולא ואיתימא רבה בר בר חנה אמר ר' יוחנן כמחלוקת לענין טומאה כך מחלוקת לענין שבת אבל לא לענין חליצה

seeing that I could change it from the right [foot] to the left?<span class="x" onmousemove="('comment',' A sandal had two strappings, perhaps like loops, through which the laces were inserted, one on the outside and the other on the inside of the foot. Now, if the inner one is broken, it can be mended, and though it is not very seemly to walk in sandals with the strappings or laces merely knotted together, nevertheless it does not matter, as it is not very noticeable on the inner part of the foot. But if the outer one is broken, one would not walk out in it until a new one is inserted; consequently it ceases to be a 'utensil', and may not be handled on the Sabbath (cf. p. 125, n. 3). In Abaye's case the outer strap was broken, hence R. Joseph's ruling. But Abaye argued that by changing the sandal to the other foot this would become the inner strapping, hence it should be permitted. Presumably their sandals were not shaped exactly to the foot, and were interchangeable. ');">²¹</span> — Said he to him: Since R. Johanan explained [the law] on R. Judah's view, it follows that the <i>halachah</i> is as R. Judah.<span class="x" onmousemove="('comment',' That it ceases to be a 'utensil' if the outer is broken. ');">²²</span> To what does this refer? — For it was taught: If the two ears of the sandal<span class="x" onmousemove="('comment',' At the back, by means of which the sandal is held when it is tied up. ');">²³</span>

Jerusalem Talmud Shabbat