Indulge me, please, just one more time. (Fixes redundant and broken 
button handling.)

Ken

local fn ForwardReverse( whichWay as str15 )
dim as integer i, a, b, nlines
dim as handle  efH

a = 0 : b = 0

appearance button 2

select case whichWay
case "reverse" : gDirection--
case "forward" : gDirection++
end select

if gDirection =<  1 then gDirection =  1
if gDirection => 16 then gDirection = 16

if gDirection =  1 then a =    1 : b =  499 : appearance Button 2
if gDirection =  2 then a =  500 : b =  999
if gDirection =  3 then a = 1000 : b = 1499
if gDirection =  4 then a = 1500 : b = 1999
if gDirection =  5 then a = 2000 : b = 2499
if gDirection =  6 then a = 2500 : b = 2999
if gDirection =  7 then a = 3000 : b = 3499
if gDirection =  8 then a = 3500 : b = 3999
if gDirection =  9 then a = 4000 : b = 4499
if gDirection = 10 then a = 4500 : b = 4999
if gDirection = 11 then a = 5000 : b = 5499
if gDirection = 12 then a = 5500 : b = 5999
if gDirection = 13 then a = 6000 : b = 6499
if gDirection = 14 then a = 6500 : b = 6999
if gDirection = 15 then a = 7000 : b = 7499
if gDirection = 16 then a = 7500 : b = 8000

gC = ""
for i = a to b
long if len(dictStr(i)) > 1
gC += dictStr(i) + chr$(13)
xelse
exit for
end if
next i

edit$(10) = #gC
       efH = tehandle(10)
   IF efH != 0 then nlines = efH..teNLines%

long if FN UBound(dictStr) => 500
long if nlines = 500
appearance button 3
xelse
long if gDirection = 1
appearance button -2
xelse
appearance button -3
end if
end if
end if

gC = ""
edit field 0

end fn