I found the low frequency surprising as it's so easy to pronounce--I suppose tc is used in most cases. Here's what I found for bigram freqs near TK:

Ratios (count / total) and percentages:

    PG: 0.00047%
    TK: 0.00046%
    KK: 0.00045%
    HQ: 0.00042%
    FN: 0.00042%

Every other one here I'd expect to see: Postgres, kk/okay (and my initials), headquarters, function. Of course there's Tcl/Tk but not used nearly as much as it could.