diff --git a/slicetimer.cc b/slicetimer.cc
index ab14ef5a50de219abe3a34c7dbdd4e135076d518..f8f1ee7e364cab3f6e95139a4dbb4d32b5499b00 100644
--- a/slicetimer.cc
+++ b/slicetimer.cc
@@ -50,10 +50,10 @@ Option<string> outputname(string("-o,--out"), string(""),
 			  string("filename of output timeseries"),
 			  false, requires_argument);
 Option<string> tcustom(string("--tcustom"), string(""),
-			  string("filename of single-column custom interleave timing file"),
+			  string("filename of single-column slice timings, in fractions of TR, range 0:1 (default is 0.5 = no shift)"),
 			  false, requires_argument);
 Option<float>  tglobal(string("--tglobal"), 0.5,
-			  string("global shift (default is 0.5 = no shift)"),
+			  string("global shift in fraction of TR, range 0:1 (default is 0.5 = no shift)"),
 			  false, requires_argument);
 Option<string> ocustom(string("--ocustom"), string(""),
 			  string("filename of single-column custom interleave order file (first slice is referred to as 1 not 0)"),