Took me a while to find it, but I've dug out an article by Phyllis Kinney (1986): "One of the most striking aspects of Welsh traditional tunes is the large number of melodies which lie within a narrow compass of between three and seven notes. Of the 852 traditional Welsh songs in print, 189 (22+%) are within this range: 3-note compass = 1; 4-note = 1; 5-note = 60; 6-note = 80; 7-note = 47." Later in the article: "The fact that 22% of Welsh tunes in print are in narrow compass ... compared to about 4% of English tunes is surprising enough but that nearly one-third of the Welsh narrow-compass tunes should be pentachords is remarkable."

She later lists all the pentachords, and the more well-known are Titrwm Tatrwm , Oes Gafr Eto (along with several other Goat Counting Songs), and Ar Lan y Mor . Also, Ar Fore Dydd Nadolig which Mudcat's Mary Humphreys does so well.

Looks like compass is a cultural thing ...

sian