The presence of Eusebius's gospel apparatus (often incorrectly referred to as the 'Eusebian canons') in the margins of many of the great gospel codices is sufficient evidence of the importance of that work in the history of gospels' study. At first sight it appears an unproblematic tool: it allows the reader to note at a glance whether a point being made in one gospel's text (sometimes as long as one of our chapters and on other occasions less than a sentence) is to be found in all four, or just three or tow, gospels, or nowhere else; and then, if appropriate, to find 'the parallels.' As such it is a gospel 'harmony' which preserves the integrity of each gospel, and of the four distinct gospels in that it avoids creating a fifth text, a diatessaron.
However, the apparatus contains a series of hermeneutical assumptions about how the gospels relate to history, how the four texts relate to one another, how the four relate to 'the gospel,' how they were to be read together to provide a single composite history of Jesus, an apologetic before those who would highlight discrepancies (e.g. Porphyry), and what constitutes 'a parallel.' When we compare these assumptions wth those used today (e.g regarding 'the Synoptic Problem') we can observe the extent to which Eusebius's method determined the overall shape of gospel comparison until the 18th century whereby there was a notional complete story of Jesus of which the four actual texts were specific redactions / 'slants'.