I include this information in the note section of all of my cabled patterns, but it never hurts to have it other places too.
I write cables like this: a/b FC or a/b BC.
"a" is the number of sts placed on hold on the cable needle.
"b" is the number of sts worked before returning to those on hold.
FC means the held sts are left at the Front of the work.
BC means the held sts are left to the Back of the work.
An example: 2/3 BC translates as "place next 2 sts on a cable needle and hold at the Back of the work, K3, K2 from the cable needle".
What if the instruction says a/b FPC or BPC? Excellent question. The P stands for purl and means the "behind" sts are purled instead of knit. These can be either the a or b sts depending on the cable.
For example: 1/2 BPC translated as "place the next st on a cable needle and hold at the Back of the work, K2, P1 from the cable needle".
Or you might see 2/2 FPC. The translation is "place next 2 sts on a cable needle and hold at the Front of the work, P2, K2 from the cable needle.
Clear as mud? If you can, see if the chart shows the crossing more clearly for you. If that's not feasible, or doesn't clear things up for you, send me a message at the email address in the sidebar of this blog and I'll get back to you as soon as I can.
Happy cable crossings!