diff --git a/src/ThickNumbers.jl b/src/ThickNumbers.jl
index 08f638e..24e7e2b 100644
--- a/src/ThickNumbers.jl
+++ b/src/ThickNumbers.jl
@@ -484,7 +484,11 @@ _norm(x::AbstractArray) = sqrt(sum(abs2, x))
 Returns `true` if `isless(hival(a), loval(b))`, `false` otherwise. See also [`≺`](@ref).
 """
 isless_tn(a::ThickNumber, b::ThickNumber) = isless(hival(a), loval(b))
+isless_tn(a::ThickNumber, b::Number) = isless(hival(a), b)
+isless_tn(a::Number, b::ThickNumber) = isless(a, loval(b))
 Base.isless(::ThickNumber, ::ThickNumber) = throw(FPTNException(isless, isless_tn))
+Base.isless(::ThickNumber, ::Number) = throw(FPTNException(isless, isless_tn))
+Base.isless(::Number, ::ThickNumber) = throw(FPTNException(isless, isless_tn))
 
 """
     a ≺ b
diff --git a/test/runtests.jl b/test/runtests.jl
index ddab14c..872e4ae 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -45,13 +45,18 @@ using IntervalArith
     @test_throws FPTNException Interval(1, 2) ≈ Interval(1, nextfloat(2.0))
     @test !isapprox_tn(Interval(1, 2), Interval(1, 2.1))
     @test isless_tn(Interval(1, 2), Interval(nextfloat(2.0), 3))
+    @test isless_tn(Interval(1, 2), nextfloat(2.0))
+    @test isless_tn(0, Interval(1, 2))
     @test_throws FPTNException isless(Interval(1, 2), Interval(nextfloat(2.0), 3))
+    @test_throws FPTNException isless(Interval(1, 2), nextfloat(2.0))
+    @test_throws FPTNException isless(1, Interval(1, 2))
     @test Interval(1, 2) ≺ Interval(nextfloat(2.0), 3)
     @test_throws FPTNException Interval(1, 2) < Interval(nextfloat(2.0), 3)
     @test Interval(nextfloat(2.0), 3) ≻ Interval(1, 2)
     @test_throws FPTNException Interval(nextfloat(2.0), 3) > Interval(1, 2)
     @test !isless_tn(Interval(1, 2), Interval(1, 2))
     @test !isless_tn(Interval(1, 2), Interval(2, 3))
+    @test !isless_tn(Interval(1, 2), 2)
     @test Interval(1, 2) ⪯ Interval(2, 3)
     @test Interval(2, 3) ⪰ Interval(1, 2)
     @test issubset_tn(Interval(1, 2), Interval(1, 2))