diff --git a/.gitignore b/.gitignore
index c38fa4e0..851d0de2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 .idea
 *.iml
+coverage.txt
diff --git a/sub_test.go b/sub_test.go
new file mode 100644
index 00000000..8e26d416
--- /dev/null
+++ b/sub_test.go
@@ -0,0 +1,92 @@
+package xop
+
+import (
+	"errors"
+	"github.com/muir/xop-go/xopnum"
+	"github.com/stretchr/testify/assert"
+	"testing"
+	"time"
+)
+
+func TestLog_Settings(t *testing.T) {
+	defaultLog := Default
+	logSettings := defaultLog.Settings()
+	assert.Equal(t, DefaultSettings, logSettings)
+}
+
+func TestSub_StackFrames(t *testing.T) {
+	sub := Default.Sub()
+	sub.StackFrames(xopnum.DebugLevel, 100)
+	sub.StackFrames(xopnum.ErrorLevel, 100)
+	sub.StackFrames(xopnum.TraceLevel, 2)
+	assert.Equal(t, 100, sub.settings.stackFramesWanted[xopnum.ErrorLevel])
+	assert.Equal(t, 100, sub.settings.stackFramesWanted[xopnum.AlertLevel])
+	assert.Equal(t, 2, sub.settings.stackFramesWanted[xopnum.TraceLevel])
+	assert.Equal(t, 2, sub.settings.stackFramesWanted[xopnum.DebugLevel])
+}
+
+func TestSub_SynchronousFlush(t *testing.T) {
+	sub := Default.Sub()
+	sub.SynchronousFlush(true)
+	assert.True(t, sub.settings.synchronousFlushWhenDone)
+	sub.SynchronousFlush(false)
+	assert.False(t, sub.settings.synchronousFlushWhenDone)
+}
+
+func TestSub_MinLevel(t *testing.T) {
+	sub := Default.Sub()
+	sub.MinLevel(xopnum.ErrorLevel)
+	assert.Equal(t, xopnum.ErrorLevel, sub.settings.minimumLogLevel)
+}
+
+func TestSub_TagLinesWithSpanSequence(t *testing.T) {
+	sub := Default.Sub()
+	sub.TagLinesWithSpanSequence(true)
+	assert.True(t, sub.settings.tagLinesWithSpanSequence)
+	sub.TagLinesWithSpanSequence(false)
+	assert.False(t, sub.settings.tagLinesWithSpanSequence)
+}
+
+func TestSub_PrefillText(t *testing.T) {
+	sub := Default.Sub()
+	assert.Empty(t, sub.settings.prefillMsg)
+	sub.PrefillText("text")
+	assert.Equal(t, "text", sub.settings.prefillMsg)
+}
+
+func TestSub_NoPrefill(t *testing.T) {
+	sub := Default.Sub()
+	assert.Nil(t, sub.settings.prefillData)
+	assert.Empty(t, sub.settings.prefillMsg)
+	sub.PrefillAny("foo", "bar")
+	sub.PrefillText("text")
+	assert.NotNil(t, sub.settings.prefillData)
+	assert.NotEmpty(t, sub.settings.prefillMsg)
+	sub.NoPrefill()
+	assert.Nil(t, sub.settings.prefillData)
+	assert.Empty(t, sub.settings.prefillMsg)
+}
+
+func TestSub_PrefillBool(t *testing.T) {
+	sub := Default.Sub()
+	sub.PrefillBool("key", true)
+	assert.Len(t, sub.settings.prefillData, 1)
+}
+
+func TestSub_PrefillDuration(t *testing.T) {
+	sub := Default.Sub()
+	sub.PrefillDuration("key", 1*time.Second)
+	assert.Len(t, sub.settings.prefillData, 1)
+}
+
+func TestSub_PrefillError(t *testing.T) {
+	sub := Default.Sub()
+	sub.PrefillError("key", errors.New("error"))
+	assert.Len(t, sub.settings.prefillData, 1)
+}
+
+func TestSub_PrefillFloat64(t *testing.T) {
+	sub := Default.Sub()
+	sub.PrefillFloat64("key", 64)
+	assert.Len(t, sub.settings.prefillData, 1)
+}