The two principal series in the Willsie Collection are seal matrices and royal charters with seals. The seal matrices are made of bronze, copper alloys (especially latten), and lead, and date from Romano-British of the 2nd-3rd centuries CE until the end of the 15th century. There are also a few papal bullae and some examples from the 16th-18th centuries. Matrices can be round, oval, vesica-shaped, or lozenge-shaped, usually with raised handles ending in a suspension loop. A linen cord, leather thong, or metal chain would have been inserted through the loop in order for the matrix to be worn around the owner's neck. Most of the later medieval seal matrices have simple images, such as a cross, star, flower, or animal. Personal seals bear the names of owners or heraldic arms, flowers and animals, and religious images well known in Christian iconography (e.g. Agnus Dei, Cross, Pelican in Her Piety, Virgin Mary and the Christ Child). Among those 14th-century seal matrices with Latin, Anglo-Norman, and Middle English mottos are the following: box 4, no. 4, "Vei ci parti" ["Behold my lawful action"]; box 4, no. 11, "Pense de bien fere" ["Think of good deeds"]; box 4, no. 14, "S[igillum] Secreti Amoris" ["Seal of secret love"]; box 4, no. 20, squirrel with motto "I cracke notis" ["I crack nuts"]; box 4, no. 22, "Lel ami avet" ["You've got a friend"]; box 4, no. 26, "Lege tege" ["Read, keep secret"]; box 4, no. 27, "Le lion ici dort" ["The lion sleeps here"]; box 4, no. 28, "Love me and ei ye" ["Love me, and I thee"]; box 5, no. 35, "Hel lovely ladi"(?) ["Hail Lovely Lady"(?), i.e. the Virgin Mary]." Concerning "I cracke notis" and other Middle English seals and charters in the Manuscripts Division, see Don C. Skemer, "Cover Note," in Princeton University Library Chronicle (Spring 2014).

English royal charters are the largest series, with examples from King John to Elizabeth II. Most of the charters dating from the reigns of King John to King Henry VIII have been are mounted for preservation and display purposes. Other charters are not mounted unless so indicated. Most of the charters have two-sided pendant seals, generally attached to the document by means of a parchment tag or braided silk cords. Most depict the king seated on the throne (obverse) and most often the king as mounted knight (counter-seal). These are supplemented by royal seals no longer attached to documents.