<p>One of Rome’s most ancient monuments, the Tigillum Sororium was a beam supported by two vertical posts that spanned a road (Festus 380: <i>sororium tigillum</i>; Dion. Hal., <i>Ant. Rom.</i> 3.22.7-9), probably the *Sacra Via (Richardson; contra, Coarelli). Dionysius of Halicarnassus relates that the beam served in the expiation of Horatius and that located nearby were two altars, one to Iuno Sororia and the other to Ianus Curiatus (<i>loc. cit</i>.). The Arval calendar reports that the Tigillum Sororium was located AD COMPITVM ACILI (<i>CIL</i> VI 32482, of Augustan date; Degrassi, <i>Inscr. Ital</i>. 13.2, 37, 515; *Compitum Acili); thus its location is fairly secure, despite the absence of archaeological remains.</p>