>If I use an incrementing number as a surrogate primary key, why should I make it a character field? Tastrade does that, and I've heard that it's the thing to do. But a key like this is not useful for ordering the records, and therefore would not be part of compound index expressions. Wouldn't it be better to use a double field?

Old foxpro stored numeric field as character internally, therefore evaluating of index expression would just take one more internal step, and also character field (as you mention) had obvious advantage for constructing compound keys.
Visual Foxpro integer filed is not charater-like anymore, and actually it takes less bytes, therefore internal processing will be even faster. So, VFP prefers integer for surrogate primary keys.