It was reassembled out of five fragments. The left fragment was found in 1886 in the so-called "Korai Pit" northwest of the Erechtheion. The exact place and date of the discovery of the rest of the pieces remains unknown.

It is an anta capital with painted decoration once placed on a tall pillar. The capital supported four small bronze and marble dedications, fixed on corresponding sockets on the capital’s upper surface. In one of the sockets, a bronze plaque with two holes to secure the legs of a bronze statuette is still preserved.

On the capital’s front there are two inscriptions carved at different dates and highlighted with red and green colour. The oldest one reads: ΟΝΕΣΙΜΟΣ ⋮ ΜΑΝΕΘΕΚΕΝ ⋮ ΑΠΑΡΧΕΝ ΤΑΘΕΝΑΙΑΙ ⋮ HΟ ΣΜΙΚΥΘΟ HΥΙΟΣ, meaning “Onesimos, Smikythos’s son, dedicated me to Athena as an aparche”. Later on, higher up on the capital the second inscription was carved by a different hand. It reads: ΘΕΟ[ΔΟ]ΡΟΣ ⋮ ΑΝ[ΕΘΕΚΕΝ ⋮ ΟΝ]ΕΣΙΜΟ ⋮ H[ΥΙΟΣ], meaning “Theodoros, Onesimos’s son dedicated it”.

It is unclear if Theodoros’s inscription was accompanied by the addition of a new dedication, or was simply carved on the occasion of restoring his father’s dedication that was destroyed. It remains also uncertain the time difference dividing the two inscriptions. In any case they were both carved before the Acropolis was destroyed by the Persians in 480 BC.

Onesimos is identified with a well-known Athenian vase-painter whose name appears on eight perirrhanteria (water basins) found on the Acropolis. One of these (ΕΜ 6543) is kept today in the Acropolis museum storerooms whereas the others are in the Epigraphical Museum of Athens.