Perfect consonance interval - Kanzenkyowaontei

...The degree of consonance is thought to be higher when the frequency ratio between the two tones is simpler. The simplest frequency ratios of 1:1 (perfect 1st) and 1:2 (perfect 8th) are called absolute consonant intervals, and adding 2:3 (perfect 5th) and 3:4 (perfect 4th) to these, they are called perfect consonant intervals. 4:5 (major 3rd) and 5:6 (minor 3rd), as well as 3:5 (major 6th) and 5:8 (minor 6th) are imperfect consonant intervals, and all others (major and minor 2nds and 7ths, various augmented and diminished intervals, etc.) are called dissonant intervals.

*Some of the terminology explanations that mention "perfect consonance interval" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…協和の程度は，2音間の振動数比が単純なものほど高いと考えられる。振動比が最も単純な1：1(完全1度)と1：2(完全8度)の場合を特に絶対協和音程といい，これに2：3(完全5度)と3：4(完全4度)を加えて完全協和音程という。4：5(長3度)と5：6(短3度)，および3：5(長6度)と5：8(短6度)を不完全協和音程，これ以外のすべて(長･短の2度と7度，各種の増･減音程など)を不協和音程という。…

※「完全協和音程」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報