#include<bits/stdc++.h>
#define ll long long
#define all(a) (a).begin(),(a).end()
using namespace std;

const int maxn = 1e3 + 10;

ll n,m;
ll cnt2,cnt3,cnt4;
ll col[maxn][maxn];
void work1()
{

    cnt2=4;
    cnt3=(n/2-1)*2+(n-2)/2*2;
    cnt4=(n-2)*(n-2)/2+1;
    bool flag=0;
    int x,y,z;
    for (int i=0; i<=cnt2; i++)
    {
        for (int j=0; j<=cnt3; j++)
        {
            int k=(m-i*2-j*3)/4;
            if (k>=0 && k<=cnt4 && i*2+j*3+k*4==m) {
                x=i; y=j; z=k;
                flag=1;
                break;
            }
        }
    }
    if (flag==0) {
        cout << "Impossible\n";
        return;
    }
    cout << "Possible\n";
    if (x==1) {
        col[1][1]=1;
    } else if (x==2) {
        col[1][1]=1;
        col[1][n]=1;
    } else if (x==3) {
        col[1][1]=1;
        col[1][n]=1;
        col[n][1]=1;
    } else if (x==4) {
        col[1][1]=1;
        col[1][n]=1;
        col[n][1]=1;
        col[n][n]=1;        
    }
    ll q,p;
    q=3;
    while (y) {
        if (q>=n) break;
        col[1][q]=1;
        y--;
        q+=2;
    }
    q=3;
    while (y) {
        if (q>=n) break;
        col[n][q]=1;
        y--;
        q+=2;
    }
    q=3;
    while (y) {
        if (q>=n) break;
        col[q][1]=1;
        y--;
        q+=2;
    }
    q=3;
    while (y) {
        if (q>=n) break;
        col[q][n]=1;
        y--;
        q+=2;
    }
    p=2,q=2;
    // cerr << z << "\n";
    while (z)
    {
        col[p][q]=1;
        q+=2;
        if (q>=n) {
            p++;
            q=p%2+2;
        }
        z--;
    }
    for (int i=1; i<=n; i++)
    {
        for (int j=1; j<=n; j++)
            if (col[i][j]) cout << "R";
            else cout << "B";
        cout << "\n";
    }
}
void work2()
{
    cnt2=2;
    cnt3=n-2+n-2;
    cnt4=(n-2)*(n-2)/2;
    // cerr << cnt2<<" " << cnt3 << " " << cnt4 << "\n";
    // cerr << 
    bool flag=0;
    int x,y,z;
    for (int i=0; i<=cnt2; i++)
    {
        for (int j=0; j<=cnt3; j++)
        {
            int k=(m-i*2-j*3)/4;
            if (k>=0 && k<=cnt4 && i*2+j*3+k*4==m) {
                x=i; y=j; z=k;
                flag=1;
                break;
            }
        }
    }
    if (flag==0) {
        cout << "Impossible\n";
        return;
    }
    cout << "Possible\n";
    if (x==1) {
        col[1][1]=1;
    } else if (x==2) {
        col[1][1]=1;
        col[n][n]=1;
    }
    ll q,p;
    q=3;
    while (y) {
        if (q>=n) break;
        col[1][q]=1;
        y--;
        q+=2;
    }
    q=2;
    while (y) {
        if (q>=n) break;
        col[n][q]=1;
        y--;
        q+=2;
    }
    q=3;
    while (y) {
        if (q>=n) break;
        col[q][1]=1;
        y--;
        q+=2;
    }
    q=2;
    while (y) {
        if (q>=n) break;
        col[q][n]=1;
        y--;
        q+=2;
    }
    p=2,q=2;
    while (z)
    {
        col[p][q]=1;
        q+=2;
        if (q>=n) {
            p++;
            q=p%2+2;
        }
        z--;
    }
    for (int i=1; i<=n; i++)
    {
        for (int j=1; j<=n; j++)
            if (col[i][j]) cout << "R";
            else cout << "B";
        cout << "\n";
    }
}
void solve()
{
    cin >> n >> m;
    for (int i=1; i<=n; i++)
    {
        for (int j=1; j<=n; j++)
            col[i][j]=0;
    }
    if (n % 2==1) work1();
    else work2();

}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);cout.tie(nullptr);
    int t = 1;
    cin >> t;
    while(t--) solve();
    return 0;
}