Abstract. Truncation-error analysis is a reliable tool in predicting convergence rates of discretization errors on regular smooth grids. However, it is often misleading in application to finitevolume discretization schemes on irregular (e.g., unstructured) grids. Convergence of truncation errors severely degrades on general irregular grids; a design-order convergence can be achieved only on grids with a certain degree of geometric regularity. Such degradation of truncation-error convergence does not necessarily imply a lower-order convergence of discretization errors. In these notes, irregular-grid computations demonstrate that the design-order discretization-error convergence can be achieved even when truncation errors exhibit a lower-order convergence or, in some cases, do not converge at all.1. Introduction. These notes are a response to the recently published article [18]. The article applies a truncation-error analysis to evaluate accuracy of finitevolume discretization (FVD) schemes on general unstructured grids. The analysis is accompanied by computations performed on regular and irregular grids. While we agree with the analysis and its conclusions in application to regular smooth grids, we consider the truncation-error analysis and some of the conclusions derived from it invalid in application to irregular-grid computations.On regular grids, convergence of truncation errors (also referred as local errors in the literature) is an accurate indicator of convergence of discretization errors (also referred as global errors). However, the truncation-error convergence is often misleading for FVD schemes defined on irregular (e.g., unstructured) grids. As shown in [18] and before this in [17], the second-order convergence of truncation errors for some commonly used FVD schemes can be achieved only on grids with a certain degree of geometric regularity. Other studies, e.g., [3,6,7,10,13,14,15,16,20,21], showed that truncation-error convergence degradation on irregular grids does not necessarily imply a degradation of discretization-error convergence. Examples shown in the following sections confirm that on irregular grids, the design-order discretization-error convergence can be achieved even when truncation errors exhibit a lower-order convergence or, in some cases, do not converge at all. Note that these results do not contradict the Lax theorem, which states that consistency (convergence of truncation errors) and stability are sufficient (not necessary) for convergence of discretization errors. In fact, for some formally inconsistent discretization schemes, it has been rigorously proved that the discretization errors converge [3,6,7,13,16,21].Article [18] applied a truncation error analysis to FVD schemes for the Poisson equation. A thin-layer approximation was analyzed. It was shown that the truncation error is O(1) (i.e., does not converge) in grid refinement unless the grids are regular. The discretization error of the scheme was inferred to be non-convergent. By coincidence, the particular thin-layer FVD s...