diff --git a/CodingStandards.md b/CodingStandards.md
index 4938506..cf5e3b3 100644
--- a/CodingStandards.md
+++ b/CodingStandards.md
@@ -38,6 +38,7 @@ from allmydata.util.assertutil import _assert, precondition, postcondition
  * Feel free to ignore the part of PEP-8 that says to put each module import on a separate line, but don't import modules from multiple separate packages on the same line.
  * Ignore the part of PEP-257 which says to put the trailing `"""` of a multi-line docstring on a separate line separated by a blank line.  (That rule appears to have been motivated by a limitation of Emacs which has been fixed.)
  * Ignore the part of PEP-8 which specifes 79- or 72- char line widths. We use 77 columns. In emacs set `fill-column` to 77 (e.g. `M-x set-variable fill-column 77`).
+ * PEP 8 says: "If operators with different priorities are used, consider adding whitespace around the operators with the lowest priority(ies)." This is a good rule; note that it also applies to some non-obvious low-priority operators, like '`:`' for list slicing. (Example: `a[: d]b-c` good, `a[- c:d]b` bad. If a slice is from the start or to the end of the array, put the '`:`' immediately next to the bracket on that side.)
 
 ### truths and falsehoods