What is a critical spine and when should I avoid it?

Question

Recently, I made my first forays into professional book cover design. I submitted a design for a glued-back book with a base white front and back, but a coloured spine.

The customer, an experienced publisher, replied that they were averse to risk doing what they called a 'critical spine'. As they have more knowledge in this field than I do, I trusted their gut and made the spine base white as well. Ended up fitting the design better, anyway.

Now my question is: what are the risks of such a 'critical spine': a spine with a markedly different colour than the front and back of the book? I understand that a sloppy printer may misregister the print with the folding and thus bleed the spine onto either the front or the back (or both, if spine width was incorrect). But are book printers that inaccurate? Are there other factors that I need to keep an eye on with a 'critical spine'?

In what situations should I avoid a critical spine, and when is it fine to use it?

Answer 1

My guess: their bindery equipment cannot guarantee to fold/score exactly on the line between your colored spine and the white cover(s). Same thing applies in three-fold brochures. I call it a "design element on fold." It requires hairline bindery precision, which is rare if not impossible (which is why we have margins & bleeds).