To measure your dog for a Martingale Collar, use a soft tape measure or a piece of string and take the measurements at point A, B, and C in the drawing below.  Choose the size that fits within this range to ensure that your collar fits perfectly.

For Sight Hounds, the measurement at points A and B may be the same or very similar, this is because their heads are often the same size or smaller than their necks.  For non-Sight Hounds, these measurements will be different and you will need to choose a collar size that is large enough to slip over the head (point A) but not too large to fit correctly at point B.