Tonearm Measurements

Hello everyone,
I am in process of buying a tonearm for my Oracle AC turntable (the same as MK1 or MK2 except it doesn't have electic 33 to 45 RPM switch).
I am little confused. I emailed Oracle. And they said that I should get a tonearm that "...that does not exceed 9" (armtube)..."
At the same time I use now 12" inches long Denon-401 tonearm with my turntable. And it appears if fits the turntable just fine.
I've read several tonearms manuals. And I notices there are two kind of lenght measurements taken. One lenght is being called as "Overall Lenght". And another is "Effective Lenght". "Effective Lenght" has always smaller number than "Overall Lenght". In case of my Denon-401 tonearm...
its Overall Lenght is 12.6 inches and
its Effective Lenght is 9.6 inches.
Would someone please explain what the difference between "Overall Lenght" and "Effective Lenght". And how to measure them.
And if you happen to know what lenght tonearm should have to fit to Oracle AC, MK1, MK2 turntables please let me know.
Thanks in advance!
Effective length is measured from the pivot to the stylus; overall length is from the back end of the arm tube to the end of the headshell. I am sure that they are talking about effective length.
I've just measured the distance between central spindle to tonearm's pivot point (the point the tonearm spins around) and it appears to be 9" inches (just 2-3 mm less to be exact).
I've heard that this distance is considered to be the most important measurement than needs to be taken into account while choosing proper tonearm. But I still don't know what tonearm's length corresponds to this distance....I was able to pull following info from Internet... if the distance between the central spindle to the rotation point of the tonearm is around 8.5" inches then 10" inches tonearm should be used.
If that distance is around 9.5" inches then 12" inches tonearm should be used... Please confirm if that sounds right to you.
Thanks in advance!

Before driving yourself too crazy, try to determine if an arm could be mounted at some slightly different point on your table. Being able to vary the mounting point just a few mm closer to or away from the spindle would open up a much larger field of tonearms.

However, IF your spindle-to-pivot dimension is FIXED at 225-226mm, then your table was designed for an arm longer than a Rega but shorter than a TriPlanar (just to pick two examples I'm familiar with).

Regas have a stated mounting dimension (spindle-to-pivot) of 223mm, although 219.5-220mm works better. A TriPlanar has a mounting dimension of 233.5mm. Obviously Oracle had something between those two in mind.

A Rega has an effective length of 240mm. A TriPlanar has an effective length of 250mm. For the mounting dimension you stated you'd need something in between.
.....or a Triplanar and a Skill saw.....