Since I mentioned the higher expense ratio and someone else asked about it, here's an example:
Vanguard Total Bond Market, ETF class : "BND", expense ratio 0.05%, minimum investment 1 share (currently $79.46 / share)
Vanguard Total Bond Market, admiral class : "VBTLX", expense ratio 0.05%, minimum investment $3,000
Vanguard Total Bond Market, investor class : "VBMFX", expense ratio 0.15% (closed to new investors)
If you only have $2,000 to invest in bonds, then you're left with "BND" (ETF class). If you have $3,000 or more, you can pick either ETF shares ("BND") or admiral shares ("VBTLX"). It looks like VBMFX is no longer available.