What constitutes a “substantial part” of a copyrighted work is an endlessly diverting question – both because copyright law tends not to draw “bright lines” (meaning that lawyers are forever doomed to conducting “quantitative” and “qualitative” analyses) and because clients routinely want definitive answers which drive lawyers to distraction (“what do you mean you don’t know whether a fifteen-second clip from a movie infringes copyright?”). Canadian courts have been remarkably (but understandably!) coy in fashioning their interpretations of that portion of Section 3(1) of the Copyright Act which states that copyright means “the sole right to produce or reproduce the work or any substantial part thereof“. The Copyright Board’s recent decision (released May 23, 2015) in the Access Copyright (Provincial and Territorial Governments) 2005-2014 tariff provides some examples of a decision-maker assigning some numbers to the notion of a “substantial part”.
“Substantial part” could be seen to operate as a euphemism – in some ways it’s simply a substitution for “non-trivial”, “important” or “significant”. Thus, the Supreme Court of Canada in the CCH v LSUC (2004 SCC 13) decision stated, in its discussion of fair dealing, that where “the amount taken from a work is trivial, the fair dealing analysis need not be undertaken at all because the court will have concluded that there was no copyright infringement” – “substantial part” operates as a threshold matter for whether we are in the realm of copyright infringement at all. If something less than a “substantial part” has been copied, the copier has not encroached on the exclusive rights of the copyright owner in the first place. The concept of “substantial part” bears impressive gate-keeping powers. It is usually wielded via impression: the Supreme Court itself has described it as “a flexible notion … a matter of fact and degree”, to be decided “by its quality rather than its quantity” (see Robinson v Cinar, 2013 SCC 73 at para. 26). It is thus noteworthy when we are provided with a decision which seems to indicate where we can “draw the line” so to speak in determining what constitutes a “substantial part”.
In its Access Copyright tariff reasons, the Copyright Board stated the following in the context of determining a “substantial part” of a literary work (emphasis added):
 In this matter, without the benefit of a qualitative analysis of each of the copied works, and without even knowing which portions of a work were copied, in our opinion the amounts proposed by the Consortium, being 1 to 2 pages of a work, are reasonable approximations in establishing non-substantiality. However, since 1 to 2 pages of a short work can amount to a great portion of that work, we further limit this approximation by requiring that the copying of 1 to 2 pages not constitute more than 2.5 per cent of the entire work, the percentage equivalent to what the Board had previously considered not to be substantial reproductions in its Satellite Radio Services decision.
The final reference there is worth highlighting, as it offers a lens into a different type of work – for that we turn to paras. 97-98 of the Copyright Board’s reasons for its decision in Statement of Royalties to Be Collected by SOCAN, NRCC and CSI in Respect of Multi-Channel Subscription Satellite Radio Services (6 May 2009, Corrected Version):
 Thus, the question is reduced to whether the 4 to 6 second buffer is a substantial part of an entire work. The rolling 4 to 6 seconds of a musical work is not an aggregate of an entire work. At no time does a subscriber possess a series of 4 to 6 second clips which when taken together would constitute a substantial part of the work. It matters not that over time the totality of all works transmitted are reproduced. We are dealing with a rolling buffer and at no time can we line up all of the fragmented copies amounting to one complete copy of a musical work. At no point in time can one extract from the RAM of the receiver more than 4 to 6 seconds of a song (or more accurately of a signal). More importantly, at no time is there a choice as to what goes in there or when it comes out.
 In our opinion, the 4 to 6 second buffer fails to satisfy the substantiality requirement. It is not a substantial part of the protected work.
To summarize: in the view of the Copyright Board, 4-6 seconds of the average pop song with a duration of a few minutes is not a “substantial part” of a musical work/sound recording, and 1-2 pages of a literary work falls below the substantiality threshold, so long as that 1-2 pages does not constitute more than 2.5% of the work in question. I don’t want to make too much of these holdings – Copyright Board decisions are not court decisions, and the Access Copyright decision, in particular, is qualified in the Board’s own words as being without the benefit of a quantitative analysis. The 2015 Access Copyright decision may yet be appealed to the Federal Court. Assessing the “substantial part” of a musical work is inherently going to be a much more impressionistic exercise (because the “valuable” part of the song, such as the chorus, is usually more finite than the valuable part of a literary work). But for the moment, these two decisions at least provide some indication of where the substantiality threshold might lie, or, at worst, an indication of where to begin the assessment.