There are a lot of ways that you can divvy up your device, both for visual interest and for the clearing of conflicts. Field divisions can also be used to make your charge more interesting to look at, and to help highlight elements within your device.
Some of the most basic field divisions are per pale (the field divided in half vertically), per fess (divided in half horizontally), per bend and per bend sinister (divided in half cross wise, from top left to bottom right, and top right to bottom left respectively), per chevron (a point coming up from the base), and per chevron inverted (a point coming down from the top of the device). You can see them in the image, above.
Other divisions, such as gyronny (which can have any number of divisions from six through 16), party of six, per saltire, and quarterly, are also used in period heraldry. There are many examples of a wide variety of field divisions available using search engines, Pinterest, and Wikipedia (which I rarely suggest, but they have original rolls scanned for close inspection).
There are many reasons for dividing the field of your device. The most obvious is to include more color (or metal). Dividing the field in half, in one of the ways described above, makes what we call a "neutral" field. This means you can place either a color or a metal onto it. As an example, if you wanted a blue background but the charge you prefer happens to have a lot of blue in it, you could solve the problem by splitting the field in a way that allows the blue to be on a white or yellow area. Alternatively, you may decide that you want to have a charge that is all yellow or white, but the field color you wanted is too similar to someone else's device. Dividing the field could give you the difference needed to clear your own choice.
There are some special rules associated with field divisions. First and foremost, you always need to remember the standard heraldic rule: don't put a color on a color or a metal on a metal. There's a very tiny amount of leeway for that when dealing with a divided field, if contrast is great enough, but it's definitely something that you need to keep in mind.
There's a particular heraldic thing called "marshalling", which the SCA does not use. Marshalling is when two or more devices are combined to make a single device. In medieval Europe, when a couple was married, they would sometimes present their coats of arms side by side in a "per pale" style, or in quarters, "quarterly". While you are personally welcome to display you and your partner's arms together in that way, this style of device cannot be registered. Any device that looks too much like marshalling will likely be returned. If you're considering a per pale or quarterly field for your device, have a look at the marshalling rules before delving into your design. Here's a great page with information on avoiding marshalling in SCA heraldry.
One thing you do not often see is a device that has a divided field which is all color or all metal. It is entirely possible to use both of these in most situations, provided you follow the "metal on metal/color on color" rules. This means that if your field is split argent and Or, your charge must be all color. If your field is split between two colors, then your charge must be either argent or Or. These types of field divisions can make for very dramatic looking devices, and I heartily advocate their use!
When you are in the process of creating your device, it might seem like it's going to be impossible to figure out a unique combination that is both simple and enjoyable. As a new herald, I watch all of the new devices as they are posted to our registry system, and I also read all of the posts in our heraldry forums. I am constantly seeing very simple devices, some even field primary (meaning there isn't a big picture in the middle of it), passing easily. There are still many simple, elegant devices out there!
As always, work with an experienced herald as you design your device. We work hard to try and learn all the rules, and even if we personally don't know them, we know other heralds who've been doing this for years and DO know them. Heraldry is very much a team-sport, and we help one another out on a regular basis. This means that when you work with a herald, you not only get their eyes, you likely get the eyes of several others as well!
Good luck, and happy designing!
Previous articles of interest:
Some of the most basic field divisions are per pale (the field divided in half vertically), per fess (divided in half horizontally), per bend and per bend sinister (divided in half cross wise, from top left to bottom right, and top right to bottom left respectively), per chevron (a point coming up from the base), and per chevron inverted (a point coming down from the top of the device). You can see them in the image, above.
Other divisions, such as gyronny (which can have any number of divisions from six through 16), party of six, per saltire, and quarterly, are also used in period heraldry. There are many examples of a wide variety of field divisions available using search engines, Pinterest, and Wikipedia (which I rarely suggest, but they have original rolls scanned for close inspection).
There are many reasons for dividing the field of your device. The most obvious is to include more color (or metal). Dividing the field in half, in one of the ways described above, makes what we call a "neutral" field. This means you can place either a color or a metal onto it. As an example, if you wanted a blue background but the charge you prefer happens to have a lot of blue in it, you could solve the problem by splitting the field in a way that allows the blue to be on a white or yellow area. Alternatively, you may decide that you want to have a charge that is all yellow or white, but the field color you wanted is too similar to someone else's device. Dividing the field could give you the difference needed to clear your own choice.
There are some special rules associated with field divisions. First and foremost, you always need to remember the standard heraldic rule: don't put a color on a color or a metal on a metal. There's a very tiny amount of leeway for that when dealing with a divided field, if contrast is great enough, but it's definitely something that you need to keep in mind.
There's a particular heraldic thing called "marshalling", which the SCA does not use. Marshalling is when two or more devices are combined to make a single device. In medieval Europe, when a couple was married, they would sometimes present their coats of arms side by side in a "per pale" style, or in quarters, "quarterly". While you are personally welcome to display you and your partner's arms together in that way, this style of device cannot be registered. Any device that looks too much like marshalling will likely be returned. If you're considering a per pale or quarterly field for your device, have a look at the marshalling rules before delving into your design. Here's a great page with information on avoiding marshalling in SCA heraldry.
One thing you do not often see is a device that has a divided field which is all color or all metal. It is entirely possible to use both of these in most situations, provided you follow the "metal on metal/color on color" rules. This means that if your field is split argent and Or, your charge must be all color. If your field is split between two colors, then your charge must be either argent or Or. These types of field divisions can make for very dramatic looking devices, and I heartily advocate their use!
When you are in the process of creating your device, it might seem like it's going to be impossible to figure out a unique combination that is both simple and enjoyable. As a new herald, I watch all of the new devices as they are posted to our registry system, and I also read all of the posts in our heraldry forums. I am constantly seeing very simple devices, some even field primary (meaning there isn't a big picture in the middle of it), passing easily. There are still many simple, elegant devices out there!
As always, work with an experienced herald as you design your device. We work hard to try and learn all the rules, and even if we personally don't know them, we know other heralds who've been doing this for years and DO know them. Heraldry is very much a team-sport, and we help one another out on a regular basis. This means that when you work with a herald, you not only get their eyes, you likely get the eyes of several others as well!
Good luck, and happy designing!
Previous articles of interest:
Edgithe's device, currently going through commentary for registration. This is a fairly good example of a two-color field division (per bend sinister) with a gold (Or) charge on top. See how clear the hawk image is? It really pops, and has great contrast. That's what you're after!