Herald's Notebook: Don't confuse "in [a position]" with "on [a thing (and ordinary or charge)]".

Take the example of "chief" if you sat that something is "in chief" you mean it is at the top of the shield. But "a chief" is a thing, an ordinary, and being a thing it needs further description, a tincture (color) at least. Thus we have "a chief gules". If the chief itself is decorated with a charge, then we say either "a chief gules charged with…" or "on a chief gules".

"On" is for when one thing, a charge, is layered on top of another. "In" is for when you are specifying which position a thing is in.

It relates to why it is always "in chief" but "on a chief", the chief ordinary, being a thing, we need to specify not only its color, but how many of it there are. Ordinaries are always singular, charges may be plural. The position "chief" is not a thing. it is just a description of somewhere on the shield, the "top" of it, if the knight is holding the shield in front of him, the part closer to his head, the place of importance as opposed to "base" or the "base point".

We don't say "top" because it is ambiguous. While a shield is being made it lies flat on the table where it is being made and we then layer areas of paint and metalic objects one on top of another acording to the directions in the blazen. When we're done and it is held upright things that were on top as we built it are now in front and the chief is on top.