Based on their format, the Greek catenae can be divided into three classes: marginal, two-column and alternate. The first class is the most common: the biblical text, written in larger letters, is at the center of the page, flush with the inner margin; Richmond Subway Map around it are arranged, in the three outer margins, the exegetical extracts more rarely, the extracts fill all four margins and the scriptural text is perfectly centered on the page. In two-column catenae, the sacred text and the extracts face each other in two symmetrical columns. In alternate catenae, the biblical text, which is set off by diple marks that are repeated in the margin of the manuscript, is followed by the exegetical extracts, uninterrupted on the entire page. Similar to the marginal class is a type of catena which is constructed within a commentary in another already-existing catena. For example, MS Reg. 44, at the center of the page, attests a commentary of Hesychius on the Psalms, and in the margins, a catena of fragments on the same biblical text; the beginning of Palat. 20, at the center of the page, contains a catena on Luke, and in the margins fragments of Nicetas on the same gospel. In the oldest catenae, the authors of the extracts are generally indicated, usually in red or in more conspicuous characters, their names wholly written out or sometimes abbreviated, which can here and there lead to errors.