Illustrative example
Someone in a three-way business with no written agreement writes to the other two saying they are leaving. The others reply that everyone verbally agreed to give it five years, so the notice does not count.
Section 26 applies to a partnership at will - one where no fixed term has been agreed - and there any partner may determine it at any time on notice, without a reason and without anyone's agreement. The fact that decides whether the section applies is whether a fixed term really was agreed, because an agreement for a five-year term would take the firm outside it and into sections 32 and 35 instead. The evidence for that is what the three actually did, not what one of them now recalls.
The three agree the firm is dissolved from an agreed date, that the two continuing partners buy the leaver's share at a valuation, and that the business carries on under a fresh written agreement between them.