Pixel spacing is the distance from the center of one light to the center of the next, usually given in inches. It sets how many lights end up on your roofline and how the line looks from the street.
The math is simple. Twelve inches divided by the spacing gives lights per foot. At 6-inch spacing you get 2 lights per foot, so a 100-foot roofline holds 200 lights. At 12-inch spacing you get 1 light per foot, or 100 lights on the same run. At 4-inch spacing you get 3 per foot, or 300 lights.
Closer spacing gives a smoother line. From the curb, lights a few inches apart start to blend into one continuous glow, especially in white. Animations look smoother too, because a moving color has more steps to pass through.
Wider spacing gives a more traditional look, closer to classic holiday bulbs, where each light reads as its own point. Some people prefer that, especially on older houses where a string-of-bulbs look feels right. Patterns with two alternating colors also read clearly at wider spacing, since each color gets its own distinct dot.
Viewing distance matters. A house set far back from the road on a big lot can carry wider spacing, because distance blends the lights. A house close to the sidewalk shows every gap, so tighter spacing tends to look better there.
Spacing has to survive corners. On a hip roof or a complex roofline with several peaks, the installer has to keep the gap even as the line turns. With very wide spacing, a light can land awkwardly short of a corner, which shows at night. Tighter spacing hides those small mismatches.
Peaks and rakes are where spacing shows most. A gable outlined with evenly spaced lights reads as a sharp triangle. Uneven spacing near the top makes the peak look lopsided.
More lights per foot means more pixels to power and control, so power draw rises with tighter spacing at the same brightness setting. It also means more points for a pattern to use. A chase or a twinkle effect on tight spacing looks fluid; on wide spacing it looks like lights blinking in turn.
Ask what spacing is being quoted and to see it lit at night from about the same distance your house sits from the street. If a system offers more than one spacing, ask why one is recommended for your roof.