It's just to the left of the a/b button.  It's the second from the left 
in that row and is a bit odd sized.  If pressed when not in split, it 
just gives a boink, so it helps confirm if you are in split or not.

73, Steve KW3A

On 10/25/2011 9:51 AM, Steve Dresser wrote:
> Curt,
>
> Holding down the A/B button sets the two VFOs to the same frequency.  I know
> which button you're talking about, but I don't think it's labeled "A/B".
> Worth checking before you do the tour.
>
> Steve
>
> ----- Original Message -----
> From: "Curtis Delzer"<[log in to unmask]>
> To:<[log in to unmask]>
> Sent: Tuesday, October 25, 2011 09:07
> Subject: Fun on 10 meters
>
>
>> well when I do my "tour of the 590s" I want to briefly describe as
>> though I sound like I know what I am talking about, these features,
>> as well as things I have discovered such as; when you hold down the
>> a/b button to check the split frequency, you can actually tune the
>> main knob while holding down the button, which is truly neat! and
>> having the speech not speaking at that time is much better than
>> having to click the mic button a tiny bit to stop it speaking all the
>> time, etc.
>> thank you, Steve! a lot for the explanations. This is truly a fantastic
>> rig.
>> I started out with the most humble of gear, a DX35 Heath transmitter
>> and Collins 75A1, tank of a receiver, (weighed 52 pounds) and was
>> solid, though not real sensitive.
>> comparing what was then to now, is like science fiction come to life
>> in many ways.
>>
>> Curt
>>
>> W B 6 H E F
>>
>
>
>