Rhythm deserves its own section because it applies to both piano and guitar.
Sections
17.1 Pulse
27.2 Beat
37.3 Tempo
47.4 Meter
57.5 Time signatures
67.6 Subdivision
77.7 Syncopation
87.8 Swing
97.9 Strumming patterns
107.10 Counting systems
Rhythm path
11. Find the pulse
22. Count 4/4
33. Count 3/4
44. Count 6/8
55. Quarter notes
66. Eighth notes
77. Sixteenth notes
88. Rests
99. Ties
1010. Dotted rhythms
1111. Syncopation
1212. Swing feel
1313. Strumming patterns
1414. Piano accompaniment patterns