Here's how it's used:
* -graph: Used for things that *record* or *produce* a picture.
* Electrocardiograph: A device that records electrical activity of the heart.
* Radiograph: An image produced by x-rays.
* -gram: Used for the *picture* itself.
* Electrocardiogram: The tracing produced by an electrocardiograph.
* Radiogram: The image produced by a radiograph.
Examples:
* Electrocardiogram (ECG): A recording of the heart's electrical activity.
* Radiograph (X-ray): An image produced using x-rays.
* Sonogram: An image produced using sound waves (ultrasound).
It's important to note that while "-graph" and "-gram" share the same meaning of "picture", they are used in slightly different contexts. "-graph" refers to the device or process used to create the picture, while "-gram" refers to the picture itself.